Sounds like the trans may be starting to slip. What year and how many miles on the trans? How's the fluid look? Nice and red or brown/black (wipe the dipstick on a white towel to be sure of the color)? Might be time for a pan drop and fluid/filter change along with a quart of Lucas stop-slip. Would also be a good time to install the Trans-go shift kit to help firm it up and hopefully make it last longer.