First, I should warn you that a reset will initialize all the user data, which means there will be no voice patches loaded when you are done.
That's ok, I have the original voice cartridge.

I don't own a DX7 myself, but I've heard that pressing and holding down the "Edit", "16", and finally "32" buttons and then releasing them all will get you to the reset.
Unfortunately, that's not working on my DX7.

I've also heard that similarly pressing Function/16/32 provides access to diagnostics.
Well, that's working, but isn't giving me any needed information.

The DX7 has a lithium battery used for RAM back-up purposes. The don't usually last more than about 10 years, although sometimes a bit longer.
The battery is ok, "battery check" function gives 2.8 Volts

I'm not familiar enough with the DX7 to know if strange behavior could be related to a weak (or leaking?) battery, especially if the user data isn't being lost when the synth is off.
Could that be one way to reset the device? If the battery is removed, all the data in the RAM should be erased...

One solution to the problem could be the E! board and it would also give some extra functionality to my dx7. Unfortunately they are very hard to find these days, I assume.

Maybe I'll try to remove the battery, unless someone advises me NOT to do so...

Thanks!