Heat not working good all the time?

Firewall, also known as forward bulkhead. Not really called firewall anymore by manufacturers for liability reasons. It's the front metal structure that encloses tr passenger compartment. Also serves as the mounting point for components like steering shaft, brake booster, and multiple smaller components and wiring. Don't worry about someone giving you a hard time. Not everyone has the knowledge base, and those that do didnt at one time. Learn as you go.

 

Thank you. I'm familiar with and engine but not a genius. I don't do my own work but I like to know what's going on and what's what

 

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Don't worry about it. We all started somewhere. Eventually, you'll learn enough to do your own work with confidence. First, MAKE SURE your coolant level is full.
On the heater control valve, the brackets that holds the cable housing (tube the cable slides through) sometimes become loose which could cause the housing and cable to move in a manner which doesn't open the valve. It is possible that this could cause an intermittent lack of heater control. There is one bracket on each end of the cable. One on the firewall, and one on the back side of the heater control itself.
Without getting hands on, it's harder to figure out what's going on. So that is the easiest thing to check. As stated above, have someone move the control on the dash (cold-hot-cold-hot) to check if the valve and cable are operating correctly.
If the engine itself is not warming up, there are other issues. It sounds simple, but air and water movement. Something to do that is simple would be to get a no-contact thermometer (hand held type that looks kinda like a barcode reader). Drive to or from work or wherever, pop the hood, measure the temp of the radiator at the inlet and the outlet, measure the temp of the block at the waterpump and the thermostat housing. Also measure the top of the thermostat housing (I'm assuming that is where the temp sensor is) and base to see if there is a variation (this would indicate an air bubble which could give false temp reading). Last, measure the temp on the block at the expansion plugs. This is to check temp of coolant that is actually in the block.
If the engine and radiator are both excessively cool after warming it up, it would indicate that you have uncontrolled flow through the radiator. That would most likely be a thermostat that is failed in the open position or temp rating is way too low. As I stated before, in very cold conditions, people cover the grill (DO NOT COVER THE RADIATOR ITSELF) to reduce airflow which allows for a quicker warmup and less temp fluctuation while driving.
If the radiator is cool but engine itself is hot, you have coolant flow problems. This could be caused by low coolant level, failed/failing water pump, air trapped in the coolant system.
Sorry if I seem to be rambling on, trying to get as much covered before my meds kick in.

 

Since your engine temp isnt.getting.up to normal operating temp surly it isnt anything to do with any of the heater components like the valve on the firewall or any of the cables or any of that stuff for that matter. Unless just coincidentally your temp guage is whacked out the same time you're having low heat situations, highly doubtful though! Restricted heater core would seem a likely culprit but again that wouldnt affect the engine running temp. I know you said the tstat was changed, twice, but gosh it sure seems like that bitch has failed and is stuck open just gushing all that super cold coolant through your poor heat deprived engine. Other than that idk... as mentioned already i would pry get yourself a cheap IR thermometer(they're not to spendy these days and they are kinda cool to have and you'll find yourself using it on all kinds of other shit, or just playing with it lol!) and actually see what the temps are and where. Another idea would be to get yourself a scan gauge or the like, which, hell everyone should have one or two of those things anyway! Ill never not have one of them things again! Being able to monitor all those parameters(like water temp in your situation!!!) and run, check and clear trouble codes right there whenever you need or want is priceless, atleast to me! They just do so much more too, awesome lil tools tnat make life easier... But yeah you could track your water temp with that too, right down to the exact degree, instead of the dash guage that if working correctly might be within 15 degrees of proper running temp when the needle is where its usually at there in the middle er whatever.... the IF thermometer would be able to track temps in various places though which could be more beneficial. Another option could be that there is an air pocket somewhere, but again highly doubtful since surly it would have worked its way out and it prolly wouldnt cause low engine temp. But air certainly causes weird temperature issues so...?? An air pocket would cause low cabin heat however when in the heater core! I would check that dang tstat once again, especially if you didnt use an oem one!!! The part store ones are typically hella cheap compaired to a toyota one, i bet youd be suprised in the difference. Plus you'll notice they will be a completely different design even though youd think a tstat is a tstat but its not the case. Forgive me if you already know all this stuff, just throwing stuff out there for you or even others reading this in the future when these first gens will be full on antiques! You can test them too in a pot on the stove aswell to confirm for yourself they are working correctly......

Idk....keep us posted my man!

***Lol wholly shit! Sorry for the novel! Damn! My feelings wont be hurt if you dont make it all the way through my post haha!