'98 Ford Escort ZX2

I've got about 115,000 miles on the car, but dont' know the engine size. About a year ago it started to misfire at the higher RPM range. I've had the plugs, wires, and fuel filter changed. I've run a ton of fuel injector cleaners through it but it still misfires. When it first starts, it runs fine, then after a little while (5 or 10 minutes) it starts to miss at the higher RPMs, and slowly gets worse, untill it starts to die out when idling. It seems to miss more often when accelerating or letting off the fuel, but not as often when just driving steadily. It used to only miss when burning regular unleaded fuel, but has started to do it when burning the higher octane fuel also. When it does it on the interstate, I can usually hit the clutch and turn off the car for 10 or 15 seconds then pop the clutch and it "resets" the problem. I'm a college student so I dont' have a ton of money to troubleshoot and the best the dealer can give me is that it's misfiring (as if I didn't already know that). Any ideas?
Thanks in advance,
Matt