There is much discussion about the AC being "dirty" so as suggested I
may only be treating a symptom. The surge protector that the problem
computer plugs into is shared by a monitor and speakers. Would it be
unusual for only the PC to be effected by the dirty AC?

No Not unusual at all. Reason is because a PC Power Supply "generally" is
partially powered at all times if plugged into a HOT outlet.
"generally" Meaning depending on Design.

Ever See the Ones with the 1 / 0 Switch on the back? Those are most
Certainly (in my experience) almost always partially powered on even when
the PC is Shut Down by say initiating an OS Shutdown at the Software level.
Sometimes you will even be prompted that its now ok to shut down the PC, and
you go ahead and press the power button on the Front of the case - but the
1/0 button is still on , meaning the PS is Still running , but the DC side
is OFF...as well as the PC.

So any "dirty" AC power can enter the PS still on the AC side and 'degrade'
Components so that next time you turn it on, it will appear to be acting
strangely.

Now, I dont know how the KB Soft Start Cicruitry works myself, but I suspect
that perhaps a small amount of DC , probably in the Milliamps as described
by Steve, is pulsed into the MOBO as a 'keep alive' to detect press of a
key. Given what I just Explained, it DOES make some sense that one could
come to the conclusion that the Soft Start Jumper is "drawing too much
power" if you look at it like that.

The Design on those Supplies could differ than the ones W-O soft Start,
(which I think is that other Small Square Molded Connector?) Wheras the DC
side of thePS could be "Partly" energized at a very low level and even be
misinterpreted by the MOBO as AC , NOT DC.....which also will cause
Electrolytics to "****" excuse the term. There could also be poor
Electrolytics on the MOBO Of Course, but like I say , where theres Smoke (or
in this case Electrolyte) theres Fire or in this case Dirty power.


I will have to double check but I believe the circuit that feeds this
system is surge protected at the panel. Would that prevent the AC
from being dirty?

You may want to Either put the 1/0 switch to 0 after its Shut down, or Just
get one of those multi Power Centers where you can switch off each item so
its not always HOT.

The Protection on a typical cheapie Power Strip wont protect against the
Unusual, only the typical. Like I said Earlier, we are in Times in this
World where Electricity Demand is at an all time high. Dont think the East
coast USA blackout was just a glitch....if there was enough power to go
around , they wouldnt be borrowing it from each other.