OK, FWIW the outcome of this was...
I ran some memory tests but nothing much came up, and VI kept crashing no matter what I did.
So I gave up on that machine, and took out my extra 2Gb of memory and installed it in my other Giga machine. Removed the GS3 memory tweak, added the /3GB switch, loaded VI and Forte.
And it works like a dream.
21 instances of VI, with 1527 samples loaded - and still over 1Gb of free memory. Clearly on this box CPU will top out before memory does.
As to why it didn't work on the other machine, I have no idea. I'm just grateful that I happened to have another reasonably beefy box standing by as a spare.
Mike