On Wed, 23 Feb 2011 16:42:44 -0800, Jeff Liebermann
wrote:

Another day in bad caps hell. I fish a Dell GX520 out of the pile at
the local recycler and proceed to try and raise the dead. I've done
this many times with good success, but not this time. Upon initial
inspection, I find the usual bulging electrolytic caps. There were 4
identical caps, 2200uF 6.3VDC 105C. I replace them with 2200uF 10VDC
105C caps (Panasonic ECA-1AHG222). Note the increase in working
voltage. These are brand new caps from Digikey. I run the unit
overnight and find that two of the caps now have bulging tops.
(snip)

Duz anyone have a clue as to what is causing this problem? I haven't
had time to probe the board for power supply bus noise or excessive
current quite yet. What I could use is some speculation as to the
probable culprit.

Jeff, I've been reading your posts long enough to conclude you know
what you're doing, generally in spades.

But ... did you check the polarity? I ask that because there are
cases where the PCB screen legend was arse-about, causing grief to
cap-replacers who didn't record the "outgoing" cap polarity and match
that against the screening.

Otherwise, if heating isn't an issue then what WW said is a
possibility.