1991 Toyota pickup, start it, runs great until comes to temp then seams to drop a cylinder and lose power and run like crap. Truck has 340,000 miles on it but I did a complete rebuild 75,000 miles ago. I checked valve clearance and a few other things like new plugs and wires, air filter, oil change but all seamed well. Didn't sam to improve anything.

 

There is a water temp sensor for the ECU on my 86 22RE, so maybe there is one on your 91?