1998 Mercury Mystique air conditioning goes "cold-hot-

My AC will not consistently stay cold, but it is capable of getting cold, just completely inconsistent. Some days it will never get cold at all. Other days it will be cold for 15 minutes, then go warm and never go back to cold again on that same drive.

There have also been times that it will go back and forth continously throughout a 30 minute drive. Stayin cold for 5 minutes, then going warm for 1 or 2 minutes, then back to cold. You get the idea.

The only thing I have done thus far is to charge the system, which did not seem to change anything, which may also mean that it didn't need charging.

Thanks for your input!