There are a few things that could actually lock up your pute'.
One is heat. Of course you say that the highest you seen it was 40 degrees. Make sure your thermostat cable is connected to your CPU in the correct place. Check CPU documentation for proper installation or seek info at AMD.
Two : Make sure you are running the correct RAM ,AMD CPU's are very fussy with which RAM they run with.
Three: Make sure you are running the latest drivers for all your hardware.
Four: to tell you the trueth ,, I had the same trouble with Windows ME ,, This is why I switched to Windows 2000. Sence the switch I've had No lockups and less headaces. Not sure how Win2k is running on AMD tho.. You'll have to seek more info.
That's all a can remember right now,,, Unless you can get more specific about things that are related to the lockups. Like Time running ,, when you run certain things ,, what version drivers are you runnig for your Video card..
Lots of stuff. Oh yah.. If you running Mcaffee make sure re-install it if you ran the upgrade. Mcaffee has been know to have some issues with WinMe. Diable it and try it first to see if it makes a differance..
Other than that these were some of my experiences.
Hope that helps.
Good Luck..