Hey so I have 2005 grand prix gtp compg. Recently it started slowing the engine down at higher RPMs. I can't tell if it is misfiring or if something is slowing the engine down, what happens is if i'm going at full throttle it does just fine all the way through first gear, about 95% of the time it does perfect in second, but when it hits third great then about fifty percent of the time it will start to symptomize misfiring. I can't tell if it is misfiring because when it does it the RPMs will drop about 400-500 RPMs, when the a/c is on this percentage increases to about 80%. This does not throw a code when it does this it only will flash the check engine light, I've tried to run a history scan but can't find anything related to it. I've had four different people tell me four different things and nothing fixed it the dealer tools me it was simply a tune up, didn't fix it, another mechanic told me it was a communication error between the transmission but I still didn't fix it, another told me coil packs, changed them, nothing. Last one I was told o2 sensors misreading. Please help me
