Please Help!!

My 1992 Geo Metro idles up, then down, then up again while warming up. Once warm, it struggles to idle. The timing belt was changed to months prior to the problems appearance. Some of my buddies think the timing was thrown off and is causing the problem. But others think it is the throttle position sensor. I need advice.

My 91 3 cylinder did that. Is the check engine light on? Mine was caused by a loose hose on the intake manifold. I also changed the pcv valve at the same time but I'm sure it was the hose that caused it. Look for all the small hoses going into the intake manifold. Mine was in the back. Also check to make sure none a crimped or rotted.