2002 Chevy Venture transmission and gas gage

Every once in a while the transmission will seem to diengage in 1st gear the engine will start to rev. If you let up on the gas it shifts into gear. The fluid and filter have been changed. This happens randomly and on happened 3 times in 1000 miles. I just purchased it. It goes into gear real good and with your foot on the brake giving it gas it does not slip. Also the gas gage acts irratic all of a sudden. This is the lowest we let the tank get so far. It started around 1/4 of a tank. I will have to fill it to see if the problem is at a low tank only. The gage just fluctuates.I have only had this for 3 months so I have no vechical history. It runs fine and check engine light has not and is not on.