Why does my car blows cold air for like 10 seconds and then it stops for no appearent reason?

The problem started a month ago whn the car was blowing cold air and all of a sudden it stopped. I thought that it might be the freon. So I bought one of those refill cans and it seemed that the problem was fixed. A week went by and the car just stopped blowing cold air again. So then I thought there was a leak in the system, I checked the system pessure and indeed it as very low, so I bought one of those "stop leak" cans today and I refilled the system with freon, but the problem now seems to be that the compressor turns on an then off when the ac knob is on. I can even feel that the car shows a slight "resistance" when the compressor goes on, and then a "relieve" when it turns off. Please hep. Thanks.