Anybody else see strange slowdown in q3 with 5.25?

[Genom]

5.13 drivers work about as well as I would expect, giving me a solid 34.7fps at my res and detail level. Hoever, if I put in 5.21 or 5.25, whenever a smoke trail apears, BOOM, frame rate dies. But if there's no smoke trails it zips along nicely. Can someone tell me if they've seen this? It's driving me nuts.

This is a G400 non max, win98se. If I uninstall and put 5.13 back in they work like a charm.