Problem solved!!!
It turns out I had a copy of 3.50 stashed with my backup copy of the PSS 777. Once I reverted back to that version, my sim is now stable and working. Just to be sure this WAS the issue, I reinstalled 3.53 -- result was CTD again 5-10 mins into the flight. Now running 3.50 again and steady as she goes on a coastal flight :-)
I suppose it's the same scenario as Graphics drivers on some folks systems -- newer is not always better. It works now though with the older module, so I am happy :-) I custom built my own rig, so I am going to presume this was entirely system related (which I knew from the beginning, since no one else posted here about this problem, this was definately something which is related directly to my system and NOT Flight Simulator).
<<>> - just got done with a 3 hour flight with FSUIPC v3.50 - no issues at all and everything worked beautifully. Problem definately solved. :-)