Well, do I feel silly.
I finally realized that I could, and probably should have been using NVagp (which does support my AMD 761) rather then AGPGART which doesn't (hence my options agpgart agp_try_unsupported=1).
Swapping was easy - Option "NvAGP" "1" - in XF86Config-4.
As my problems started with a new kernel (and hence new agpgart) I'm hoping this fixes things for me. I'll be back if it doesn't.