1998 Ford Contour

The charging system light intermittantly comes on. Battery is good, alternator is good, any ideas?

How good is good? A few simple tests at a shop could find some cheap parts malfunctioning, or even some expensive ones. Then you will have a new or working old alternator.

lose belt?

HOw about a bad connection? It is most likely something between the voltage sensor and the battery.

Alternator brushes may be worn. Take a look at the alternator when the car is running and it is dark. If you see any sparks it may be at the end of its life. Most likely this is not the problem, but easy to check.

How do you know they are good?? I would suggest you should start by checking BOTH ends of all battery cables.