it seems, that
Option "NvAgp" "0"
in the Device Section of XF86Config helps. It works with the static incompiled agpgart too. /proc/driver/nvidia/agp/status tells only "disabled" and it works for the whole day now ;-)
glxgears still renders with ca. 1100fps.
I hope it is a continiously working solution, thx for the tip Andy
AMD K6-2 500MHz
256 MB SDRAM
GeForce 2 MX 32MB
SuSE 8.1, mostly selfcompiled 2.4.19-4GB