My game lags whenever I'm in BG's or RDF

My client runs everything just fine, I have all graphic features on "Low" even though I could have them higher, and when I was levelling I never had issues with lag or low FPS. Now I've reached lvl 80 on Icecrown, and I tried a bg and rdf, and was lagging and had had pretty bad FPS on the rdf, but it was still playable (kinda). On the bg, on the other side, it was litterally impossible to play, I was at like 1 frame per 10 seconds, I couldn't even leave the bg, so I had to shut down wow externally. Anyone knows how to fix this? Thanks is advance!