Air Conditioner not working properly?

Since the end of last summer my car’s A/C hasn’t been working right. Some days it might stay on for 10 minutes, then even though nothing has been changed, it will put out warm air for a few minutes. Somedays it changes back and forth multiple times, other days (typically the hottest days) it will be mostly hot air and the odd minute or two of cold.

I did take it to a shop, and he hooked it up to a machine to check the freon and gauges - he said that the level was perfect and said everything seems to be working fine, except it hits a point and then stops the system.

I’m at a loss because he didn’t think it was the compressor or electrical, and I don’t have the money to have someone spend hours with it just to find out what’s wrong. Any advice would be appreciated!