G200 & 3d-Studio DOS refreshrate problem

[Kunta]

If I install a driver newer than the 5.20 i'm not longer able to get a refreshrate of 75Hz (it defaults to 60Hz) under 3d-Studio (full screen DOS app under Win98). I am using Vbext extension! and it used to work ell.
What is going wrong??
G200 8MB SG and Microscan 17 monitor.