ReWarp only works on Voodoo and some extent R100 and R200 cards. This is due to Warp3D not totally being backwards compatible (or the old Warp3D programs being buggy as Hans says).
Initially even the R100 and R200 froze at certain functions but with the patches of Alain most of the stuff is now working with the old Radeons. There are still artifacts in e.g. Shogo with an old Radeon.
I had a jittery mouse recently in my A3000. The heat of various cards (most notably the Voodoo3) and the various warming/cooling cycles of turning the machine on and off caused some chips to get unsettled. I pressed on the Denise chip and now the jittery movevement is gone.
Denise is also responsible for some video functions like doing color lookup.