TNT cards are generally very stable it may be that there is an IRQ conflict with another device. What is it showing in your control panel> system > device manager settings.
Some graphics cards do not like to share IRQs with sound or network cards.
The other obvious thing is when the TNT card was installed were the drivers for your old graphics card removed properly. You can check on your current card by removing the drivers and changing the graphics adapter to a standard PCI VGA card. If there are no problems with this then it is probably not your graphics card. Re-install the TNT drivers and see how it goes.
512 PC2700 DDR
Athlon 1800+ XP
Radeon 64mb DDR Vivo
IBM 46 GB
A wee Rally !