CMOS RAM failure, slowdown, CPU usage when plugged in

I have a Dell Laptop D620 running Windows XP with 3GB memory.  When it is plugged in it runs very slow, CMOS RAM fails a diagnostic program and CPU usage in Task Manager Processes is about 130% when added to System Idle Process.

When I UNPLUG it, three things happen.  The speed accelerates dramatically, the CMOS RAM passes the diagnostic test and the CPU usage in Task Manager goes from an unexplained total of Idle plus Usage of 130% to the correct 100%.  When I plug it back in, all three things go back to the way they were. I've tried it several times.

The power supply is the correct PA-12 65 watt.  BIOS has been updated.  Complete power down has been accomplished (including disconnecting the CMOS battery and holding the power button down for 30 seconds).  The CMOS battery has been replaced and the BIOS time updated.  The windows XP system has been replaced with a clean install twice.

Any ideas why this is happening and how to fix it?

Thanks!