I'll carry on my private messages with Dave L here.

I'm afraid it's looking like a hardware fault or corruption of the internal flash memory.

You need to hold down C#3, D#3 and C#4 and switch on. This will give 12 pages of comprehensive tests. Do all the integrated circuit tests on the first few pages and the later pages.

In desperation you could also overwrite the flash areas of custom memory, home page, favourites and user midi settings from the initial data disk.

Apart from that it's a bench test with a competent technician.