Prophet-5 problem

[IOTech]

I'm still working on this...

Since I really have little idea what can cause it, I tried a
shotgun...
Replaced Z80, ROM, nvRAM, dynamic ram, all logic between data bus and
d/a converter. Plus the converter itself. 
Problem still there....
As said, only one parameter is converted as if one or more of the MSB
are stuck and it's always the same parameter. All other parameters in
the synth seems ok.

I don't think it's caused by bad contacts anymore.
Sometimes the synth works ok after power-on, and it can then stay that
way for a day without the problem reappearing.
But usually the problem is evident after power-on, and it stays that
way also...

Anyone who has an idea what can be the possible cause for this?

Could it theoretically be a power-on initialization issue?
Can bits get stuck in the dynamic ram by bad initialization?
I have checked the reset circuit and it appears to be working ok.
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Ingar, Norway.