My old one did that too. Really, the only solution that worked for me, was to just use a different enclosure. My 3rd enclosure finally worked fine.
Its amazing how crappy they really are.
I now use a transcend storejet 20gig. Pricey, but only use 1usb port. It's a 1.8" drive, so no replacing it with a bigger laptop drive. I did find a 2.5" ensclosure that did work though.
If you're interested in a storejet, let me know. I have an extra one.