I have a MPS airshifter with on-board compressor now. When I turn on the compressor and build up the pressure to about 105 psi, it will stay there for about 3 or 4 hours and then it slowly creeps down to around 60 psi over a period of a couple of days, but it will stay at this pressure for weeks. Does this sound right/normal?

Yes. You only need it to stay up for 8 seconds if your drag racing.

Thanks Mav--but 9 seconds in my case :)