SP808EX freezes during start-up. Blank screen after restart

[readymedia]

Hi here!

I have a SP808 with a blank screen. Its seems to be a power / voltage issue, because if the SP808 is unpowered for some long time (a day or two) it gets to the boot animation (e mix studio also an updated version) , sometimes to SCSI check (OP1 is installed)... an then the screen freezes. But the Zip Drive continue to spin.
But after re power the unit I just see the blank screen. I uninstalled ZIP 250 drive and also the OP1 extension, tried to disconnect one after another all the cables from the mainboard... nothing helps.
PSU itself delivers +5, +15, -15. The most capacitors on the mainboard shows 5,1 Volt volt one 4,9 Volt (because there is a 100 Ohm limeting resistor).

Update: The Zip250 drive works in an old WinXP PC without problems.

What can be a problem in this case?
Any help appreciated!

Best!