Yukon xl looses speed
Hello. We own 2001 yukon xl 2500 with 160000 ml on it. Just replaced exhaust pipe and
both manifolds, did oil change and transmission fluid, car runs great but after we go on
interstate for a ill bit and stop it starts shifting weird, rough, usually from 1 to 2 gear and
from 2 to 3 I guess. It don't happen when we start it at first, only after we drive on
interstate for a while. And while going 70 mph it drops the speed to 55 for few seconds
and then picks it back up itself but for those few second we floor it all way and it just
either looses speed or stays on 55 mph. Annoying since there is no codes at all...
Nobody knows what's up. Thank you