You probably can't.
Since you already run at 2X, that's not an option dropping from 4X. That helps sometimes.
Dropping to 1X would really make that card drag along. I have one in my file server and it's not the most powerful thing on the market already.
Most cards can handle 87MHz on the AGP bus, but that's recent cards.
Time for a new mobo or new card that can handle higher bus speeds...
EDIT:
WELCOME TO THE FORUMS!
Didn't notice you were new to us. Hope you stick around...you've found the best one on the net...