When's the last time that you had the freewheel off? Spin-on freewheels will sometimes tighten themselves a little on the hub the first time that you crank up a steep hill but that usually only takes once to get it tight. If you have prawls skipping I'm thinking it's probably time for a new freewheel. Unfortunately, I'm thinking that good quality ones are probably getting progressively more difficult to find. Good luck.