For drivers, there are a few utilities that will work. The last one I used with success was this:
http://www.guru3d.com/content_page/guru3d_driver_sweeper.html
When you go to add/remove in control panel and remove a program, it just removes "surface" files but leaves of lot of things behind. Something like this, when ran, will go through you system directories and registry and remove all old trace files. It should be pretty straight forward. After removing the drivers, hunt down the latest catalyst drivers and install for your OS version.
Voltage control may not be available for all cards. What you will have is the Catalyst software for your card and it should look like this under the performance tab:
http://www.legitreviews.com/images/reviews/1585/ccc-6790-idle.jpg
I think the default clocks for your card are 840 MHz for the core, and 1050 MHZ for the memory. Click the padlock icon to unlock, and take the slider for the GPU core and move it left and try lowering it from 840 to 820/810 and click apply in the lower right - done. Overclocking would obviously be the opposite. This does not harm/void anything if under-clocking, it actually puts less stress and demand for power consumption on the card. For more advanced tweaking and possible voltage control, most would use something like MSI afterburner. If you have voltage control and Catalyst didn't list it, it would look like the slider at the top of Afterburner like this:
http://i1034.photobucket.com/albums/a427/MrDomRocks/GPUOverclockto580specs.png
Checking for stability, try this program. It's fairly common and is a good test because it isn't a synthetic test but rather a real-world kind of test. Just run it a few times and see how your system runs and note any behavior like you have been having.
http://unigine.com/products/heaven/
I can tell you for a fact that if the drivers are indeed good, when I overclock a card in stages slowly at a time, I will encounter a driver stopped responding message if I do not encounter a lockup first. What fixes this almost every time is adding voltage (within reason) because the GPU/memory cannot sustain itself on such little power supplied. So maybe this will at least give you some ideas and hopefully a free solution for now.
*Edit* - of course any diagnosing is trial and error so do things in stages instead of all at once.