Function keys not functioning

Just got a used MacBook Pro that had been running 10.6. Keys F1-9 (brightness, volume, number lock, etc.) were not working, yet F10-12 were. "Use F keys w/ fn" was unchecked in the system preferences, and I tried restarting, all to no avail.

After downloading the update to 10.6.5, all F keys were functional again...briefly. Then it was back to only 10-12 working. At one point all keys were working once again...and then back to only 10-12.

This is a bit confusing, and I'm at a loss as to how to fix it. Any feedback would be most helpful. Thanks in advance!

Tried the PRAM reset? Could be one of those simple things

Tried a PRAM reset and it didn't work, unfortunately.

I also repaired disk permissions, and after that it was working...until the computer went into sleep mode. When I woke it the F1-9 were not working, yet again.

I'm about out of ideas here!