Cylinders wont fire

2002 mustang v6 60,000milage I was going down the high way with the cruise control on at 70 start going up a incline and my car shutters and it starts acting like it wants to die check engine light starts to flash I take it to the family machanic who tells me number 5 and 6 cylinders aren't firing and he doesn't know why he says the engine is fine done a compresion test on the engine says everything checks out he is a good guy just can't figure it out please need some ideas want my car back