I think I have the problem resolved and it's just what you mentioned. On this machine, there's a conflict of some kind between WVSX and Comodo. I installed Comodo FW andI was up half the night - literally - surfing, rebooting, trying different things and then added Wisevector 3.03, running in all default mode.
A couple of reboots later, Comodo GUI was gone again, just as happened the other times. All symptoms exactly the same as the other crashes. I went into Safe mode, ran Comodo's uninstall tool and then removed WVSX. Reinstalled Comodo minus wisevector and again, no problems.
I'm thinking WVSX 2.73 doesn't conflict. I ran it with Comodo for several months. Only when I moved to v3 did things start happening. In any case, I'll give it a few more days, and if Comodo behaves, I think I can say the problem is solved. It should have occurred to me after the first Comodo crash.