As it transpired, it was the Fan-motor assembly that caused the
problems. After some 5 mins, the fan stalled and was pulling near 400mA
from the +15 supply, not enough to shut it down, but enugh to disturb
other systems.
After changing the DIL14 quad transistor array, to normal BC557's
discrete (plugged into the DIL-socket) it seems to work like a charm.
Previously I did change all the filter-caps int pwr-sup, since they
were dead.
But I think that at the moment it seems to be ok, I will leave it on
now for a day or so, to see if the fix lasts.