Vintage Synth Repair

I just joined a band where we had this tempramental old Farfisa 
Matador.  When it works it's a great light version of the reedy Farf 
sound, but the thing would produce loud hum or just stop working 
most of the time.  One day it just quit all together and didn't work 
again.

I took it home, opened it up and saw some pretty gnarly old caps 
that were starting to "explode" at the ends as well as some 
resistors that appeared to have leaked and disentigrated making the 
values unreadable.  I bought the schematic for this old beast and 
went through and replaced every part that showed visible sign of wear
(a bunch of small electrolytics in the tone section & a couple of 
resistors), and every cap(as well as a couple of resistors) in the 
power section.

A little more investigating finds two ground lines from other parts 
of the keyboard that had come unsoldered from the ground wire that 
attaches to the pinout on the tone control section.  I resolder 
those points, close the machine up and give it a whirl.  

Success!  No hum, and I got a perfectly good Farfisa sound that just 
needs to be tuned....until the volume starts to decrease after a few 
minutes and quickly there is no more volume to it.  Hmmmm...

I open it up again, discharge the caps, and poke around again - 
everything is connected properly...I marked the postive & negative 
ends directly on the fiberboard BEFORE I removed the old 
electrolytics...  I turn it on again and it works again for a minute 
or two and then the sound decreases to nothing again.  Unplug it, 
discharge the caps, plug it back in and same thing...

At this point I am thinking there is some cap in there that isn't 
functioning properly but I don't know how to tell or where to start 
troubleshooting.  I have a very basic knowledge of electronics, a 
full schematic, a good multimeter, and a broken keyboard.  

Can anyone run me down the checklist I should use to troubleshoot 
this?  I really appreciate it.  I hate to take it to a tech but if 
you guys know of a good one in Austin/Central texas I will take it 
there if no one can guide me out of the woods.  Thanks again for all 
your suggestions!