low pressure in shower?

Over time, my shower pressure decreases, both the hot and the cold (dual handle). The only way I have found to fix it is to use a garden hose with a spray jet to blast water in through the shower pipe (after removing the shower head of course) and let it run through the tub spout. Initially I tried air, and tried spraying through the tub spout, but those didn't work. After only 5-10 seconds of jetting water into the pipe, its fixed and the shower will have full pressure. Over a couple months the pressure decreases gradually and I have to blast it again. The low pressure would make the diverter squeal loudly, but I replaced the tub spout/diverter to solve that one.
I suspect that there is some kind of airlock happening, possibly related to 1) the mixer 2) the bird's nest of pipes in the crawlspace.
Background: hot runs in copper from the hot water heater, cold runs in steel from the water main. I can see the mixer from the crawlspace below, but its inside a narrow interior wall and difficult to access. Handle valves tub/shower nipples are set in tile.

Trent Botkin
January 2013