a/c question on 2003 Ford Expedition

when driving with the a/c on, it seems like it loses power (the ac). If I turn it all the way up to the highest level, it only puts out a little bit of cool air. Then after a while it starts working again.

Climate control and A/C needs diagnosis. You could have a 'swinging' blend door and it is varying the amount of cooling you receive, for example.
1 more answer
your coolant is very low, maybe a leak. But keep it full and the heat should work