MacBook Pro late 2008 hard drive problems

I've got a 2.4Ghz 15" MacBook Pro (model MB470LL/A). I've been having so many problems with upgrading the hard drive that I'm at my wits end. First, I swapped out the stock 250gb hd for a Western Digital Scorpio Blue 500gb. Only later did I find out a lot of people are having problems with it. My problem was slightly different...it would work fine for random periods of time, sometimes hours. Then the system would seemingly freeze (even iTunes stopped playing and the menu bar clock stopped running), then after maybe 15 seconds or so, it'd start up again. This started driving me nuts when it became more frequent. I tried all the different fixes from various message boards, to no avail. I even tried reinstalling OS X. No change.

So I threw that in an external enclosure (where it's been working fine since) and bought a Samsung Spinpoint M7E HM501II instead. The system seemed to crash more often, so I ran a disk scan (Drive Genius 3) on it. The scan kept either choking at a certain point and/or kicked up bad block errors like crazy. HOWEVER, when I put the Samsung in an external enclosure and ran the disk scan again, it finished without an error. I can replicate this at will.

I took it in to the local Genius Bar, where they ran more tests and determined it was a bad drive. I RMA'd it and got a new one. Same problem with the scans. And again, it runs and scans perfectly fine in the external enclosure.

So now I'm at a loss. I don't want to keep wasting money buying different branded hard drives. Is there some strange problem with my MacBook Pro? I'm thinking there some sort of weird problem with the drive controller or something, and I tried to get the Genius Bar to test that. They claimed they did and found no problem. Do I live with my 250gb drive (I'm only looking to get a bigger drive, not something faster. I need the battery life) or is there anything else I can do?