78ford4x4

I have a 1996 XJ 4.0 that runs great sometimes. The jeep will idle fine but 50% of the time while im driving it will buck and run like sh** but after a while it will be fine. This is my first jeep and i have no clue what kind of sensors it has and the local autoparts store keeps saying its a coolant temp sensor so i bought one and it was wrong one then they told me it had three and i looked can only find the one in the water neck and back driver side of head any ideas what it would be or where the third temp sensor is. PS thanks guys this forum is awsome so happy there are experinced guys whiling to help