D800 keeps slowing down

I've had trouble with this since I first got it. I know its older but I am using it as a second PC now.

I have a 2Mhz D800.  I have speedstep enabled because disabling it locks the processor speed to its slowest setting.

I have power settings configured for Always On as recommended by Dell to overcome Windows interference in Speedstepping.

I thought I might have  aheat problem but this happens with no regard for how long its been running sometimes it will run hours without this other times this will happen within seconds.

I play 3D online games.and the processor speed (seemingly randomly) slows down.  From 2Ghz often to 599Mhz (reported by Windows) and even to 179Mhz.  The games become unplayably slow.  Rebooting, cooling down for hours etc doesnt change a thing.   When I restart it will be in the slow speed still even days later.

The only reliable fix is to force reflash BIOS A13, then after rebooting its back to 2Ghz again.

It *seems* like this happens most often when the power changes from AC to battery and back.  And sometimes I get the message that it will run slower because I dont have a 90W Dell power supply attached (but I really do).

I have several Dell's and have repaired and replaced MB's and everything else - I am a bit of a Dell latitude expert and willing to try anything - this one has me totally stumped !