Just get a 3 wire to 4 pin molex and try running the CPU fan from 12 volts constantly [or use a fan with the molex connector].
The fact that you switched cases makes me think that you have managed to short something out from the mobo to the mounting plate. Try running with the mobo laying on a sheet of cardboard, completely insulated from the case.
I am running an AMD Athlon at 1 gig on an Asus A7V Mobo. Everything was working fine until I switched cases. Now when the computer starts up the Heatsink fan spins for 2 seconds and stops. All the case fans still spin and the computer continues to run for a few min then over heats and quits. Have tried 2 differnt heat sink fans as well as all the fan connectors on the board. Not post problems. No Bios Settings. HELPPPPPPPPPPPPPP?!?!?!?