Invalid File Count (Disk Utility)/slow machine. Possible RAM/HD issue?

I have an entry-level macbook (2010, white plastic unibody). Over the past year, every few months or so my system would become painfully slow (30 minute boot times, the rainbow beach ball popping up every few minutes). I would run the disk verification via Disk Utility and it would often come back with the error "Invalid Volume File Count". After booting into the OSX cd and running a repair, everything would be back to normal and the system was working as good as new. However, over the two months, this issue has been happening a lot more frequenty (almost once a week). I've ran the "Apple Hardware Test" and memtest and both came up with nothing. I decided to format the system and start from scratch. The boot time is now as fast as it was when I first bought the machine, however it's still hanging up on the most trivial processes (opening finder, typing a URL into Safari). Now that i've reformatted, I ran all of the diagnostics again (Disk Utility reports that "The Macintosh HD appears to be ok", memtest reports no issues, and neither does the Apple hardware test). Due to the number of times I had to run the disk repair, and the fact that the system is still having issues after formatting, i'm leaning towards this being an issue with the hard drive, however, all of the diagnostic tools I've tried so far want to tell me otherwise haha. Does anyone have any ideas/have experienced anything like this?