I have a 1990 Ford F150, 300 cu in 6 cylinder engine with 5-speed tranny. Recently (and after I have been driving it for awhile)...and when the weather is at or above 100 degrees..the pickup has started stumbling, bucking, and refusing to take fuel and accelerate. This only occurs when the weather is extremely HOT though, never when the weather is cool. When this incident occurs, the engine slows until I can barely tell it is running, and slowly comes to a stop. The pickup ACTS like is running out of fuel; but it isn't. The "Check Engine" light always comes on at this time. However, if I floor-board the gas pedal the pickup immediately takes off "like a scalded cat" for the short time I am floor-boarding the throttle. This incident continues until I stop and shut the engine off. When I stop, turn off the engine and let the pickup sit for 5 minutes or so, it starts up and runs like new once more!?