I've had it do that when another program pops some window up regarding some update or end of trial imminent and the like. But i've never had it do it without cause, might be worthing checking rift forums to discern the popularity of it, then hope tirion are aware of it but that its not near the top of pressing matters.
On another note, regarding your fps issue, i went old school and did some overclocking earlier, and managed to get a fair amount extra out of my processor and that alone has smoothed things up no end.