I had it occasionally with IE 7 betas and RC1, but not (yet) with IE7 final.
With the betas and RC1, it seemed random (occurred only about 4 times over a period of weeks using IE 7 daily), could not associate it with number of tabs.
Dell Dimension 8300 P4 3.0GHz (hyperthreading on), 1GB RAM, XP SP2 all updates