The first issue is with you trying to do too much from outside the main thread. Look closely at what you're doing. Without code showing what you're doing I can't tell you any more than that.
Second issue I've seen happen in the space game kit a while back I think. I don't remember ever seeing it in NGUI. Again, code / examples is key when it comes to explaining what you're doing wrong, and without any I can't do much.