it's possible the capper not crimping is driving you nuts. more likely, it's bad caps. i've ran into some kinds of caps that don't seal very well. can't name them off the top of my head, since i almost always keg, but it does happen
The cups do go bad from what I was told by my LHBS.. I just replaced the cup and also bought better caps. The ones I was using only had a small seal, not covering the whole area where it sits on the bottle. I have 3 batches that are of mixed carbonation levels due to bad caps/and cup.