Kayden : it's well-known that PJ's GFX plugins were coded on/for GF-based cards, work relatively well with Radeon cards and results are all over the place for older stuff.
If Smiff comes here, he'll tell you : "anything [ATI] older than Radeon is a shit-card"
And since Jabo's plugins are the best plugins for PJ64, this means you'll have to do like me : get used to the glitches until you upgrade the card. [or pray that ATI will release miracle drivers for R128 products... rather unlikely.]