Carl e

Started having issues with my 6l80e this morning. It takes off but once it gets to a few miles per hour it shifts itself to neutral. If I put the selector in manual mode and use the buttons I can get rolling and once it's above 10 mph I can put it back in drive and it functions fine. It will upshift and downshift no problem. It doesn't seem to slip. The truck has 172k on it with this being a reman transmission with about 38k on it.