Bad memory slot?

I recently had an issue where my son's powerbook G4 would not boot, and thanks to this forum, found that the "three beeps at start-up" indicated a memory problem. So, I removed the card and re-inserted. This worked for awhile, but when it happened again, this time I moved the card to the other (empty) slot. That seemed to solve the problem permanently. Wasn't sure if it was the card or the slot, but since he only had one 512MB card anyway, I ordered another 1GB card (so that way in case it was the card going bad, at least he could boot his machine. Well....I installed the card (in the original slot) and it didn't work. Switched the cards, and now the old card won't work. So, it seems as though the slot (lower) and not the memory card was the problem.
My question: is there any (safe) way I can perhaps clean the lower slot, so that he can take advantage of both cards? I realize it could be a hardware problem, but it did work (for awhile at least) when I first took out and reinserted the card, so I am wondering if it might not just be a poor connection caused by dirt.
Would compressed air be a good thing to try?
Any type of solvent (alcohol?) worth (or safe) to try?
I've tried pushing in the card firmly but it doesn't seem to help.