I had a similar problem with mine. The memory would get corrupted when switched off, even with a fresh battery.
In the end I found the fault was ZD1 (a 6.2V Zener diode) on the power supply board. This is used in the circuit that detects the power being switched off and puts the RAM into a safe reset state before the main power goes off. Without this happening properly the RAM can get spurious writes as the unit powers down.
Hope this might help.
