IIgs won't start up -- ZipGS dead?

[Mitchell Spector <mitch2gs@hotmail.com>]

    I'm feeling rather uneasy at the moment. :/ I was working with 
my IIgs tonight and experienced several crashes into monitor, 
freezing and and strange behavior (I've seen that before, it's not 
that unusual). I decided to power down, pop off the lid and press 
down firmly on the slot cards, including the ZipGS and its CPU 
socket connector to see if that would improve things.

    The problem is when I tried to power back up, the GS was dead!
It powers up but the screen is black and there's no system beep. 
Well, I've seen that happen too in the past, only this time cycling
the power several times didn't resolve it. So, I removed all the slot 
cards, except for the ZipGS--no change. At this point I pulled the 
ZipGS and reinstalled the original 40-pin DIP CPU and it came to 
life--not necessarily a good sign.

    To make a long story short, I tried resocketing the ZipGS in the
CPU socket (even cleaning the pins), pulled and reinstalled all its
SDRAM cache, the socketed 74xx chip, and tried swapping in two 
different oscillators. Still dead! Rather frustrating since my hardisk
just died this week and so did my ADB keyboard--only _just_ got 
that fixed in the last day or two and now this happens. Argh.

If anyone here has some speculation on what could be wrong 
or ideas, I'm opened to suggestion.

I noticed the following  incidentally...

- When I power up, the red LED on the ZipGS lights up.
- On occasion (not often) the yellow LED lights up and remains solid.
- Control-Reset causes the SuperDrive card's LED to blink.

Is it possible the 65C816 is dead, or the oscillator socket not making
a proper contact? I'll have to pull another IIgs out of storage to test
the card in there, but since the original 65C816 DIP works I doubt
the motherboard is at fault. I hope it's a socketed part on the ZipGS
and not one of the surface mount ASIC at fault. Judging by the 
symptoms, I'm guessing the former. 

This is a 15/64 configuration ZipGS, which I've been running lately
at 12.5 MHz. I hope I can get it going again, I find the machine
unbareable any slower.

Mitchell Spector