Serious graphics problems since 4/03/07 patch

[Florinda]

I don't speak geek, so bear with me here.

Since the patch last night, I've been having really serious graphics issues.

The screen doesn't refresh properly. Whenever I cast a spell, all my spell gems go black. The location dots on my minimap cause big black splotches as I move. Windows such as quest dialog windows go to black print on charcoal grey background. If I open the spells tab on my Abilities window, if I flip through pages, the print and the spell gems just sit on top of each other. If I type /flush, things return to normal until the next time I move or cast a spell or turn a page.

If there is another player in the zone with me, their toon will act as if they're in a dead zone -- I was grouped with Feinrik in Southwatch last night, he blinked at random but frequent intervals, and when he blinked his healthbar said he was 2km away. This lagged me out to the point of things looking like stop-action animation.

If I'm alone in the zone, I don't get the lag issue, but the screen refreshing thing remains the same.

Downloading and installing the latest video drivers for my graphics card did not have any effect.

Changing back to the default UI did not have any effect.

I don't know what to do and I can't do much of anything like this. Please help! (NB: if you want me to post some kind of error log, you'll have to tell me how to generate it and where to find it. Thanks.)

edit: It's happening both on Live and on Test, which implies the problem is with my computer? Why then would I have had no problem like this at all until after the patch last night?