My Suburban has 125K miles on it now and has recently developed a miss under load. I did the usual tune up things, plugs, wires, cap, rotor, fuel filter, but the problem persists. After getting to the end of my patience with replacing parts and not fixing the problem, I took it to a mechanic I trust and he has had it for a week trying to find the problem. I know he has tested the EGR system and it tested good, and he cleaned the injectors, the problem persists.
I was noticing the miss mostly under load, such as when driving at an RPM where the transmission is just about to downshift, or when driving up hills and very noticeably when towing. The miss manifests as a jerking type of power loss. Even though it feels kinda like a failure to downshift problem, the problem continues if you romp down on it and make it downshift.
Any ideas on how to fix the problem? I used high quality aftermarket replacement parts for tune up.