I had my 6l80 rebuilt about 4 months ago. We did all the upgrades zip kit new pump/bellhousing converter. Shortly after the rebuild I pulled a trailer with my 68 pickup on it around 600 miles with the tow haul on and it worked like normal. A couple of weeks after that it started to sound like rumble strips driving on the highway while the engine rpms where low (lugging the engine) I also did the thermostat change and my temperature rarely get to 140 now but it's winter but I drive 30 miles on the 75mph highway to and from work and this morning only got to 115 degrees. So it seems like it chatters or lug worse until the fluid temperature gets above 90. If I manually shift it with the tap shift and don't lug the engine I can keep the rumble strip sound and feel from happening. If i leave it in manual 5 it doesn't do it hardly at all but i get a knock code. I don't know why happened over the short time that made the change other than that was the end of summer and now it's winter and the fluid I alot colder. Cam anyone help with this? Thanks