Yes, I ran into this issue 1.5 years ago
on a Pix 535. I got the same issue that
you have except that my Pix 535 was running
on version 7.1(2). I was running Active/Standby
configuration at the time. I can ssh into
the standby Pix fine but not the Active Pix.
I opened a TAC case on this issue. It took
cisco TAC about 5 months and numerous debug,
WebEX, desktop sharing, including reboot the
Pix but problem remained. Cisco then
recommended that I RMAed the box which I found
very strange. I left the company shortly so
I never followed up with the request.
You may want open a TAC case with Cisco
for troubleshoot and RMA, if necessary.