Analogue systems

Folks,

I've got a bit of a strange problem that has cropped up with one of my RS-15
chassis.  This is an older chassis that I bought used and it is filled with
audio processing modules (filters, VCAs, comb filter, delay, phaser, etc.).
At odd times, all of the modules will produce a low 120Hz or so sound and no
inputs will work.  No other patch cords, except output, need be connected
for this to happen.  Power cycling the chassis usually fixes the problem for
awhile and then it will suddenly come back.  It is localized to just this
one chassis and all the modules seem to work otherwise.

Anybody had this happen on their system?  Any ideas? 

	Eric