lofarris

My 84 Nissan Truck with a 2.4 liter 4 cylinder will not idle after it has been running for 15 minutes or so. Last week it died on me while driving and looked like it was not getting any gas. I don't know if the fuel pump was dying or it was vapor locked but the next day it started right up, only to do the same thing 20 minutes later. I replaced the fuel pump and filters, cleaned out the carb, verified there were no vacuum leaks and added some gas treatment. It started right up and was running smoother that it has in the last 3 years. After about 15 minutes again, it died at several stoplights, but it ran great as long as I wasn't trying to idle it. I took it home and adjusted the idle rpm from 350 to 650 per spec in the manual. It is still doing the same thing, it runs great and idles very smooth for about 15 minutes then it gets real rough, puffs a lot of black smoke thru the exhaust for about 10 seconds and then dies. I can pump the gas a couple times and starts it right up. The truck only has 97k and the plugs, wires, distributor and all vacuum hoses have been replaced in the last year. Anyone have any suggestions?

Thanks in advance