parameter losing value

We have two machined with MD4-7.

In one machine a state parameter was spontaneously set to default after a restart.

The other machine lost all parameter settings, all went to default.

After setting the parameters again all was ok and the setting did not reset anymore (so far i know)

These setting can only de changed via an pin protected menu, no channels are used.

Are there any outside influences that can do something with the internal memory that can cause these problems?

Also I don't exclude tempering either.

regards, Ed