Randomly Running Hot - Help!

My '06 Taco has randomly been running hot lately. It will sometimes do that on the highway (running 35s on 4.88 gears at 70mph is about 2,700 RPM if that matters). Most days I can get away with this with no problem, but some days it spikes the engine temp (mainly on 3+ hour road trips). This has also been happening every few wheeling trips. I'll just be cruising through the woods and notice the temp climbing. On my last trip, I noticed the temp was almost in the red zone after sitting for a few minutes at a trail plug with the A/C on. I had Toyota flush/refill the radiator and do a quick once-over but they didn't notice anything too out of the ordinary. Does anyone have an idea as to what could be happening? I'd really like to get this sorted out before driving out to CO for the Jamboree. Thank you!

 

check your fan clutch. if you can spin it by hand when its off and it spins more that a revolution its toast

 

Simple and cheap, try changing thermostat. Maybe also build up of bugs and dirt in condenser.