Arghhhhhhhhhhhhhhhhhh found it.
I had added some "Solid" objects to another layer, and other stuff was bumping in to it like mad!
I'm guessing my Zoom was making the solid objects HUGE on the screen, and my objects were inside this huge new solid, and so physics went POOF.
Removed solid, back to normal...
PS Desktop C3 64x is running great!