If the power loss problem occurred just after the timing belt was replaed, there is a possibility that the belt may be installed incorrectly. If the cam timing is a tooth off from where it belongs, there will be a noticeable drop in engine power, and possibly a slightly rough or suging idle as well. I would suggest taking the vehicle back in to the shop that replaced teh belt and making them aware of the problem; ask them to verify that the cam timing is correct. If they decline, take the vehicle elsewhere and have it verified before chasing other possibilities...
I hope this is helpful; if so, an accept would be most appreciated! thanks!