1991 Ford Mustang idle and stalling problems.

I have been having trouble lately w/ keeping the car running after it the engine has running for about 10-20 minutes. The first time this happened, the car did not want to idle. It would run as long it was kept revved up. But as soon as you let off the gas it would go dead. And when you drove it and got to a cruising speed, it would buck so hard it chirps the tires. I changed out the idle control valve and and udjusted the throttle up and it idlled just fine and ran great for about a week. Then the other day, I was coming down the off ramp of the interstate and the car died. I tried recranking it and it would not start. I left it on the side of the road most of the day and when I went back to get it, it started up fine and ran great for a couple more days. Then today at the same spot on the off ramp it started sputtering and wanted to go dead. I kept it revved as much as possible in order to make it to work. It started bucking again at cruising speed in every gear and wanted to go dead. This is aggravating me because I don't know where to start in fixing the problem. The motor is a rebuilt 302 to a 306. It has aftermarket heads, intake, throttle body, mass air and has roughly between 5-10,000 miles on it. The car has run great until the last few weeks. If anyone has a clue to what might be the problem, i'd very much appreciate an answer.