Not sure why this happens

Have reseated CPU heatsink with thermal compound and run the laptop for over 24 hours runnung programs, playing DVD's etc without problems.

As soon as I connect to the internet and either download or use streaming media such as BBC Iplayer it overheats and shuts down.

If I stand the laptop up it works ok although still gets very hot but the extra air flow seems to help.

Any and all help as always is greatly appreciated.

TP