PROBLEM SOLVED!!!
So, this might seem obvious to the professionals, but it took me forever to figure out. It turns out that when X2 what scanning for VST-plugins, it was looking in both the x64 and the x86 folders (so, both 64-bit and 32-bit folders). Now, this in and of itself isn't a problem per se - in fact, I still need it to scan a 32-bit folder in order to find my Cakewalk Sound Center World Instruments expansion - but it was trying to find, and possibly run, both the 32 and 64-bit version of GPO at the same time. For some reason I was able to get it to open even while it was doing this a couple of times (seriously, two times out of a hundred attempts), but this is obviously not the way it's supposed to function.
Anyway, thanks for all the advice on here. I definitely learned a little bit about how X2 works from this whole fiasco. Most importantly, now I can composer with a little peace of mind again :)