If its an Intel CPU, then they slow down as they get too hot.
Its Intel's way of reducing the power so the temp stays safer while still
allowing the machine to limp along.
This would show up as vista getting slower and slower and using 100% CPU.
Over clocked machines can suffer from it but the owners often just blame the
software for getting slower as they use it.
You insist on blubbering on topics you know nothing about don't you
Dennis. In that regard you're just like the newsgroup idiot Frank.
I've overclocked all kinds of systems for over a decade, most Intel
NONE ran hot. Again, it depends on if you know what you're doing or
not.
For the OP, if you built your own, I'll give you odds you used too
much heat conducting compound between the CPU and heat sink. Too much
is as bad, actually worse then too little.
This system, one fairly aggressively overclocked even under stress
never gets hotter than 82F ever rendering a video all night long.