sounds like your adjustable cup might be loosening, causing the bearings to not be held in as tightly. retighten and see if that helps. if that's the problem, use some mid-grade (blue or some such) loctite next time you tighten it down. also make sure that your bottom bracket lock ring is really tight.

if it's not that, "crap" is probably getting into the races. do you have a plastic "accordion"-style sheath over the bottom bracket when you install it? also, are you using a LOT of grease? if the bearing area is saturated with grease, then it can help keep things (mainly moisture) from getting in.

and, of course, there is always the possibility that your bottom bracket races are pitted/indexed.