How can I fix my 95 Ford Mondeo s/w, fast/slow idle?

I have a 95 Ford Mondeo s/w with a 16valve 2.0 liter petrol motor with auto trans and a/c. After the car warms up (even thought the temp guage isn't working), the idle will flucuate up and down. It doesn't do this when the a/c is on. It only does it when it's warmed up and the a/c is not on. Is the problem connected to the temp guage, or idle sensor or both. If so where are these sensors located? Thanks for any help in advance. Cary