Baxi boiler over pressure problem?

I've got a baxi duo-tec he33. It's only 30 months old and has developed a problem of going over pressure.
If i run a bath for more than about 6 or 7 minutes the boiler water from the hot tap will go cold and the boiler will the start gurgling and go over pressure on the gauge before the PRV on the top of the boiler starts venting. various error codes appear sometimes it may be E168 or E131 or E125. I think this is due to where it stops in the process but not too sure. The same thing happens if i put the central heating on, 5 or 6 minutes of running fine and then it goes over pressure.
Does anyone have a clue as to where to start the fault finding process?