Well initially it sounded like artifacting from overclocked cards but then if its fine in other games apps then this shouldn't be the case. I would say that this sounds like graphics card drivers, have you tried using older drivers, just in case? You stated all of SoE's recommendations and never mentioned anything about drivers. All I will say is that I know that ATI do sometimes have problems with drivers (or at least this used to be their achiles heel) so I would look into older ones, just in case.
Oh and also know that EQ doesnt use dual/quad core optimization and so sometimes this can conflict with sound and graphics stuttering, particularly over time. If the above drivers don't help try the old CTRL-ALT-DELETE and set your processor core affinity to just one of the cores. i doubt that quad core would cause this problem but it may be worth giving it a go as it takes under 60 seconds to do. This is assuming that the quad core was also an upgrade of course. You didn't state that it was or not so added this anyway.
Oh yeah and have you tried going switching off SLI mode? As I've said before this shouldnt affect anything but with the EQ engine you just never know
Anyway good luck with it, I know how frustrating these things can be..