[SiriusHardware]

It's now sounding possible that whatever retains the programmed / variable parameters is not working in the long term, so that when the unit is powered up after a lengthy power down, the memory locations holding the stored parameters are essentially scrambled. Often these scrambled values will be not only different from those originally stored but outside the bounds of the values expected by the firmware, potentially causing some very odd effects.

Not knowing what's inside the unit and in the absence of a diagram, you are probably looking for a memory device or chip which is either backed up by an external battery or has one built in.

It's also just about possible (given its age) that it uses an electrically erasable or flash based non-volatile memory device, in which case the question about a battery is irrelevant, but otherwise:-

If the battery theory is correct then the unit should behave correctly for as long as you leave it powered up after you have defaulted it but then, if you power it off for a while (say overnight) the problem may return.

Have a look and see if there is a memory retention battery, which could be a Lithium coin cell like a CR2032, or a PCB mounted NiMH rechargeable battery, or there could be a 'thick' memory chip from the Dallas 'DS.....' series or ST 'MK....' series which includes an integral memory retention battery.

If there is a separate PCB mounted or holder mounted battery, measure the voltage, is it anywhere near the marked voltage?

If you still have the unit apart or if it is easy to get apart, could you please post some images, taking particular care to get the device numbers readable even if it means splitting the mainboard up across three or four images?