Hi. I own a 1990 Cabriolet Karman edition that is completely stock and has 150,000 miles on it. About a couple of days ago, i started my engine in the morning to head to work. I only let it warm up for a minute or so. I started to drive in to town and i had but my heater on (Its pretty cold in texas right now). I notice that the engine starts surging after about 10 minutes of driving. But after, that everything was fine when I turned off the heater. but when i got home i let my car cool down and i started it up again. and at about ten minutes the engine would start cutting out again. So i got in the car and brought the rpms up to 3000. I hit the clutch and the car dies. I fiqured it was a fueling problem, i replaced the fuel filter and checked fuel lines for collapsed lines, but it is still not working. Keep in mind that when car first starts up cold that it is running fine, but when it starts warming up it starts to die.

Any suggestions on what it might be?