This one is tricky. Im assuming you are using new batteries each time. Is it very cold? Try resetting the BIOS using the jumpers before putting in a new battery. It could be that the motherboard has a short circuit somewhere, maybe try wiping down the motherboard and the battery connector? Do you see any corrosion? I suppose if nothing else works then you can strap on some AA's or have a regulated 3V supply direct from your main car battery.