Mitsubishi PLC

I have a Mitsubishi PLC that keeps dropping it's program. We've tried replacing the CPU, replaced the battery, replacec the board that all the cards mount to, and replaced the power supply. For whatever reason though when you turn the poer off then back on if drops the program. Here's the kicker though, it'll go for a few weeks and be perfectly fine, but then it'll go for a week or two where almost every night it will drop the program. Has anyone ever had this kind of problem? it's an A1SHCPU and the power supply is A1S61P.

The panel it's in has a lot of high voltage in it (over 480V) could this be causing some sort of problem? We;ve been trying everything we can think of but it still keeps losing the program.

I've seen a faulty backplane cause "whacky" things to happen in a rack based plc. But the Battery is integral to the cpu for the A1SH. I believe there is a low battery warning (M9006?). You can write some code to turn on an output to latch something external to see if this is happening. Silly question, do you know the battery's you've put in are good?

Have you checked the earthing ?

I'm pretty sure it's not a problem with the batteries, we've put in a handful of new ones. And we've tried replcaing the back plane. Maybe we'll try doing that external latch though to see if it shows us something.

Is there a motor or something else high current that is turned off at the same time? Maybe a spike on the AC line.

I've been working with the Mitusbishi PLC's for several years and the only one I've seen drop out the program was in a panel with abour 575vac running through it and it turned out to be the memory chip.
It might be worthwhile to replace that ?? Also, we put a steel seperator between the PLC and the high voltage section. Worked for us.

You think a spike while turning it off would cause it to drop the program? There are a few large motors that I believe run off of it. And I'll have to check about the memory chip, they may or may not have tried that.