air con problem

Hi guys sorry if this has already been answered but a quick search didn't throw up anything.

I've got a 56 plate Octavia vRS TDi and im having no end of problems with the climate control making hissing noises when its turned on. It's been back to my local dealer 4 times now. It's had a new set of pipes, a new compressor and last time was re gassed as they thought air had got into the compressor. Anyway each time its been back in the problem is solved for a short space of time and then it comes back. It started again last week I wondered if anyone on here has any ideas???