kombustor doesn't stress your gfx correctly.when i overclocked my card kombustor was running well for hours but while playing crysis2,black ops it was crashing.i guess kombustor is only made for checking the temps at load.
@vivekbhujbal - heating won't be due to the back plate steel but i guess while re-fitting the new heat sink you might haven't applied the thermal paste properly !!
just try applying it again properly
also post the temp. after you start up the pc !!
@ D|C - 62C is the limit for 965BE cpus - you are using stock cooler , right ? but the temp should be a bit lower in winter season as on upcoming summer season the load temps will easily cross 62c - can you post a cpu-z screen shot and tell what app you are using to measure temps ?