My adapter is G400 DH 32SGRAM, it works well all the time running both NT4 and NT5.
You can try installing NT SP3 or later.
Can you see the information dialog under Win98 ? If can , try to setup Matrox utility to see your adapter's properties. One more question : can you play games such as doom, Quake 3...etc under Win98 ? If you can play them I guess your card should have no problem with it. Try to reinstall your NT ,it might help.