[3Rs]

I doubt if your problem is related to air in the line.  Why would the pressure be fine for a shower, then stop?

I had the same experience you described, about a year ago.  Pump worked, then didn't.  Turning power off, then on, seemed to make it work but only for a while. Called Newell, described the problem - solution:  new pressure switch for mach 5 pump.  Cost from Newell, August 2014 was $125 plus S&H.  Took 20 minutes at most to replace.  Remove 4 screws that hold the control panel cover, disconnect the small plug and unscrew old pressure switch.  Reverse steps to install new switch.  That's it!

By the way,  I have never used my city water line to fill the tank.  Never any problem with air in the tank by filling from the top, of course I don't wait until the tank is almost empty.  I refill when my water level is down to about 1/4.  Have done it this way for 3 years with no problem.  I follow John Clark's advice:  "Use it or Lose it"

Also, I kept the old pressure switch.  It is basically a micro switch with spring.  If I can find it I wiil post a picture.  

Hope this helps.  Jerry B