Sudden RPM bouncing on highway 2010 5.3l 6th speed

[Olivier Quirion]

i have a 2010 silverado 5.3L with 6th speed with 145k miles, the rpm suddendly revs up on highway at stable speed, it does it after about 20 minutes of driving at 65-70mph on flat road, it revs up 300-400 rpm for 1-2 minutes (does it also on manual mode on 5th speed (not related to afm mode)) and goes back to normal and will not do it again for at least 30 mins. Why does the tcc seems to be disengaging for absolutely no reason. (ive seen other that had this problem and it was due to misfires, but i have no codes yet)