when i start my 95 blazer it runs good at first. as it warms up it begins to idle rough for a few minutes. then it smooths back out till i go down the road. it usaully will start to miss fire around 45 or 50 mph. then it will stop for few miles an then start back up. tried changing plugs, wires distibutor and rotor, fuel injectors, fuel filter, new gas lines under the plentum, and an egr valve. still no luck with my problem?