I would pull the pan to make sure the filter has not come dislodged or that there is an over amount of debris. If the filter is intact and fluid looks good, might try the same test on flat ground and take off in m2 to see if it will duplicate. This will eliminate any fluid starvation and put a little more strain on the forward clutch to see if it is the culprit. A number of things can cause it, including worn pistons, hard seals, worn sealing rings etc.