if 50% CPU load could cause overheating I think you have a hardware problem... even 100% CPU is not supposed to cause overheating and serious problems other then lag since it eats up all your processing power.

I still believe the problem is not mIRC or it must be something quite obscure since I haven't heard anywhere here saying "Yes I experience this too" to make it reproduceable.

and you were not downloading content using the DCC feature in mIRC either? because running many DCC's could be another piece of the puzzle.

If it ain't broken, don't fix it!