Experiencing corrupted / unreadable files after download

I have a quad core mac pro running snowleopard. Recently I've noticed that I've been having more and more files be unreadable after having been downloaded over the internet. This happens both after downloading from an internet browser, downloading through a 3rd party downloader client (Continuata), as well as syncing with Dropbox.

Given the increasing frequency in which I am encountering unreadable files (i.e. when you try to open or move a file and you get a finder error that says the operation can't be completed because the file cannot be read error -36).

From what I've read this can be due to either software problems or hardware issues. From my personal experience I get the feeling I'm probably dealing with a hardware problem, but I not sure where to start. Most of my problems DO seem to occur on a single internal hard drive, but the diagnostic tests I've run speak nothing but love for the health of the drive (disk utility repair, various Drive Genius 3 tools).

Further information: I use dropbox and sync many times a day with other peoples shared folders. I also use Synergy, the mouse/keyboard sharing over the network software (and it's slightly buggy).

I've repaired permissions a bunch of times, never had any problems to be repaired. Nor have I had any problems to be repaired when I do repair disk.

Is it possible my network adapter might be going? Or some other internal hardware component on the motherboard?