ATI had notoriously patchy drivers
for their 5xxx series cards. I had endless issues with 5xxx series drivers.
IMO, a new driver
doesn't necessarily mean better performance than the previous set. ESPECIALLY for 5xxx series.
Do you remember the CAT version (ex 11.1, 11.2 etc) that you last used before updating your drivers? Maybe THAT was your sweet spot for your card. When I was using 5970s, the sweet spot for those were Cat 10.3 and 10.7.
You can download from the archive on ATI's site. Remember to download the MOBILITY drivers.
Also, ATI have a habit of not supporting "older" cards in favor of "newer" cards. ie, they will be focussing on 6xxx performance increases.
Use driver sweeper (aka post #2 above).
Hope this helps and good luck.