odd trans behavior after swap (4l60e)

finished up my swap about 2 months ago and have been driving it pretty often. hour drive to work or pulling my trailer with quads and dirtbikes. Only mods to motor installed cam and headers. working on tuning it myself. it runs great, tuning seems good based on watching long term fuel trims and narrow band gauge. wide band guage is on the list of things to get. so ive noticed once i hit around 40 or 45mph it will go into overdrive. if i am light on throttle around 1200rpm it does this weird rev up 100rpm and back down. at first i thought i had drive line vibration because it feels like its vibrating but now realize after a month of seeing this its actually reving slightly. if i give it more gas it gets to 1400 or 1500 rpm its completely fine. possible problem? torque converter failing? or maybe the torque converter lock up is below range? maybe a tuning issue? im running 33" tires with 3.42 gears maybe the decrease in rpm during overdrive put my torque converter in a bad spot? thoughts? suggestions? i think overall i plan on dumping the auto transmission for a manual but wont be till next summer. Thanks.