Grrrr this thing is frustrating me.

Two days ago, the AC would only work decent when at high speeds, did not really cool extremely well even when at high speeds, and when in stop and go traffic or putting around town it works but not well.

Yesterday, after making NO changes whatsoever, I started my car up, ac worked very well. Drove in town, very cool ac. Stop and go traffic, very good ac. Entire day the ac worked great.

What would cause the ac to perform below expectations, or almost not at all in slow moving conditions, and the next day run great at all times?