Overclocking GPU core

I can overclock my ATI 8500LE (64 MB, 3.3 ns Hynix RAM) to 275/300 w/o apparent stability problems using stock HSF on the GPU and a couple of fans on the RAM chips (no heatsinks on those). When I bump the core up even to 280, I can't even run 3DMark2001. So to my question -- would this most likely be a GPU heat issue that could be solved by replacing the factory HSF with something better? I would like to o/c to 300/300.

Thanks for any advice.

Get a crystal orb on the gpu, that kept my cards gpu cool at 320mhz, a cooljag is said to be better, that wil require some modding to fit tho i should think.

could also be a voltage problem, when you hit a brick wall like that. temp related issues are generally more gradual in nature.