Glad to be of help... it took me weeks to find that info. However....
I just noticed in the changelog for RH kernel-2.4.18-10 this change:
* Tue Jun 11 2002 Benjamin LaHaise <firstname.lastname@example.org>
- add linux-2.4.19-pageattr.patch to fix the cache attribute conflict problem
I *believe* this is a backport of the AGP speculative caching fix from the 2.5 kernel. The patch I linked to above is a workaround that works by disabling speculative caching. This patch may actually fix the architectural problems in the kernel. You may have "double fixed" your problem by patching RH 2.4.18-10.
AMD claims the workaround patch only adds a 2-3% performance hit, but you may want to try running the stock 2.4.18-10 kernel and see if you're still stable.