Tacoma running poorly, won't accelerate at all/tachometer "bounces" - what could be problem?

I'll start by saying definitely test don't guess. That said, your issues sound exactly what mine were long ago and the converter was junk (plugged).

 

Silly questions...the fuel pump and speed resistor are switched between low and high speed through a SPDT relay I would imagine.

What is the speed, low or high, of the above mentioned relay in the normally closed position?

OP said he could get to speed if he didn't get on it. That would be a lighter fuel need, imo, possibly not needing the fuel pump to be switched to high speed...

I dunno but maybe a bad pump speed relay?

 

Do not overlook the VVT solenoids. What you describe sounds very much like this, and they will *not* throw a code if they are on the way out. There are threads on how to remove/test/replace them.

 

Check for a pending P0430 or P0420.

 

Connect a fuel pressure gauge, duct tape it to the windshield and go for a drive.

Or get a backpressure gauge and see if the cats are clogged

My money is on a clogged cat because usually a weak fuel pump would code a 171/174