Oddly enough just this week received two spare diaphragms for my Collins group a dump valve, sometimes it leaked and lost boost, sometimes it jammed and chattered, sometimes it worked normal, and sometimes a bit of everything.
I took it apart, expecting to see a tiny hole, nothing to be seen, put back on ok for two days then playing up again, then ok ,then not etc etc.
It's defo the diaphragm/dump valve by the sounds of it, certainly shouldn't be releasing when stood still and level revs, my new diaphragms cost £20 for two, so not too bad.
tabetha