Transmission concerns

I recently had a used automatic tranny installed in my 2000 Plymouth Neon. It works just fine for the first 15 - 20 miles and then it shifts itself. When I step on the gas when it does this, the rpms go way up and it just keeps accelerating. I have to actually put it in 2nd to finish the drive. It has plenty of fluid. Does it just need adjusting or is it a bad tranny. The warranty on it expires Dec. 15.