car wont fire when warm

i have a 1980 chevy citation with a 2.8v6. when the car is cold it will start fine. idles a little rough but on acceleration it runs great. but when it is at operating temp and i turn it off it will not start back up until it is cold again usually about an hour wait. it cranks fine getting spark and the timing shows at 4 deg btdc. when cranking. ive tried everything i can think of. ive unplugged the choke, checked spark, battery voltage, i cant think of whats going on any help would be great