When it crashes, does your screen go black? What kind of cooling are you using on your videocard (stock/h20/etc)? What power supply are you using? It might be taxed from overclocking the cpu preventing the videocard from getting enough power. If you're using stock VGA cooling, is the fan clogged or spinning normally?
If you're using a good PSU and your cooling's working properly, reseat your VGA heatsink with some new thermal paste (although you might have to use a thermal pad as the 9700pro's generally have an annoying shim that prevents easy spacing between the gpu and hsf - the shim can be removed but it is NOT easy to do and you'll need a razor. When I had mine the stock stuff had hardened as well and I had to carefully chip it away with a razorblade. Also you can play around with ATItool a bit- might have to underclock the card some.