You could try lowering the nUI frame rate... but that likely won't help ( see this faq:
http://www.wowinterface.com/forums/s...ad.php?t=21199 )
You could also try grabbing a copy of the nUI6 5.00.00.01 (Alpha) release if you are on the nUI premium download or e-mail updates lists. I expect nUI6 to provide most people a frame rate improvement and no memory leak.