need help with hesitation problems

[down the river]

I have a 1993 Toyota 4x4 4cylinder. It has an annoying habit of intermittently losing power. It runs fine on startup. The problem is almost always noticable after driving a while on the highway and then stopping for a bit, either at a traffic light or at a rest area. The next time you go to accelerate, the engine has a diminished amout of power (maybe about 50 percent of normal). The problem disappears after maybe a minute or two of acceleration. (Usually by the time I get up to 60 mph, it is fine.) The problem is also apparent when stuck in traffic, especially on the highway. After being stuck for a while, maybe 15 minutes, the engine has only about 10 percent of its normal amount of power and will sputter and hesitate when attempting to get back up to speed. Once again, it will eventually go back to normal. I've replaced the EGR valve. I sure could use some ideas/suggestions!!!!