You can sell it if you want.
My old Voodoo 3 2000 PCI died last month but I'm keeping it just for fun.
Also, I still have a Voodoo 2 card made by Diamond. It should still work but I have not tested...
Try the suggestions about lowering the memory speeds.
If you experience the same things, then you might want to reach for your pocket and buy that Radeon! ;):D