My 85 runs just fine normally but for some reason when I drive 5+ hours the idle can get very rough. If at a stop after hours on the freeway I have to give it a little gas to keep it from stalling. Sometimes I will get a flashing or steady check engine light. Then when I get back home and go on regular drives it runs totally fine again. What could cause this?

 

what engine? can you check for an error code?