2010 Sierra 6L80e slipping 5-6?

Transmission is slipping when going from 5th to 6th gear intermittently, seems to be getting worse though. Truck has 79,000 miles on it. I had transmission flushed with factory Dex 6, filter changed and added a trucool cooler after problem arrived. Dealer won't touch truck because its lowered, headers, and tranny cooler. Truck has been tuned by Nelson about 3 years ago with no issues. Could it be electrical? No lights and no codes. Bad factory converter? Truck has never towed anything, played around on streets yes, but no track. Any ideas?

if it still under warranty take it to another dealer if not then buy you a new one...they a lil over 2000 with converter

Thanks for the reply bud8fan. I will try another dealer again. Price isn't as bad as I thought it would be.