Misfires badly when maintaining speed, but not when increasing speed.

What is your question?

I have replaced spark plugs, ignition coils, distributor, spark plug wires, MAF. It idles fine but when I try to maintain a speed in any gear it bucks like crazy, but when I step on it it goes away until I level off again. Was giving me this problem for a month or so and would also shut off after about 45 mins of driving, then it went away all of a sudden. Then out of the blue it returned. Please help! Thanks.