Checking your band tension is a good idea. Another thing that could cause that would if your front pump is wore. I lot of times racers freshen their transmissions without looking at the internal gears and flange of the front pump. Especially if your running a stock pump. If adjusting the band doesnt work, check your pressures.
At Ennis last year our dragster started doing the same thing, we went thru the transmission and everything looked great. With a gauge we could see that the pressure would drop as soon as the trans button was applied then it would roll a few inches and set, and the pressure would jump up back to 210psi. After talking to Mike Sullivan from Sullivan transmissions he gave us a new valve body, still have it in and everything still prefect.