The reason overheating causes choppy frame rates is that the CPU has been throttled right back (to a few hundred MHz) by Windows. When Windows detects overheating, it uses stop-clock throttling to slow the CPU down to levels way below the Speed Step threshold. This reduces heat output. Once the temperature has decreased a little, Windows unthrottles the CPU, so you see an improvement in performance for a few seconds -- until the CPU overheats again and Windows throttles it back once more.

If you've never cleaned it out before, the heatsink may be so badly clogged with matted layers of dust that simply blasting air through it won't be enough to clean it.  Here's a link to instructions for disassembling and cleaning the heatsink radiator grille on a 5150:

http://forums.us.dell.com/supportforums/board/message?board.id=insp_general&message.id=184349

The 5160 has an identical cooling system.

You might also want to run a fan (and therefore heat) management utility such as I8kfangui:

http://www.diefer.de/i8kfan/

Message Edited by rwm32 on 09-13-200609:37 AM

Message Edited by Leo_s84 on 09-13-200609:11 AM

My mistake: I can see that for the 5160 you need to take the palmrest off in order to remove the fan assembly and get at the radiator grille...

Here's a link to the relevant service manual section: http://support.euro.dell.com/support/edocs/systems/ins5160/en/SM/thermal.htm#wp1084976

The 5150 instructions are still worth referring to, however.

A can of 3M Dust Remover saved the day.  Everything works just fine now.  I will probably take it apart and clean it throughtfully when I get a chance though.

Thanks for all the help, I really appreciate it.