1) aftermarket may be an issue, but the top and bottom teflon o-rings are supposed to hold snuggly enough that nothing should move around. Try replacing the teflon o-rings before replacing the on/off as it could just be that they're old and a little stretched.
Also make sure that your trigger rod is adjusted appropriately, for some reason on my MicroMag, the locktite occasionaly breaks loose (once a year) and that causes me to shortstroke. When gassed up, the trigger rod should be ~1/16" from the back of the trigger (when all the way forward)(use a 1/16 allen to check spacing).
I'm really not good at writing signatures. I mean, what is it that you're really trying to do? Write something that somebody's actually going to remember, or even read? Yeah right! Nobody ever actually reads these things.