97 Ford Contour A/C problem

Having an issue with the A/C. It will run fine on the highway...but if i come to a stop at a light etc. the air will go warm after a short time. THe system needs to be evacuated and recharged but was wondering if there might be anything else to look at. One msg board had mentioned making sure the clutch gap isn't too large. (not sure what the valid range is). Any help appreciated

:banghead:

Mike