Air conditioning keeps giving up

[stripey]

On my lovely faithful green Yaris Verso ("Fun Cargo" to you guys across the pond) the air conditioning plays silly games. At start-up, it runs nice and cold for about five minutes, then suddenly stops and I just get normal air. I then turn the a/c off, keeping the blower going, or sometimes turn that off too and open the window. After 7-10 minutes I give the air con another go and it works for about three minutes. I can then keep this sort of cycle up for ever, but it's disappointing not to have it working properly.

Any one out there got any suggestions as to what might be going wrong?

Thanks, everyone.