hot water problem?

ive had my boiler for around 10 months now and not had a problem with it until now, when the shower is on it isnt supplying constant hot water like it used to its going hot for a few mins then cold then hot again around 1min later and trying to shower in it doing that is very annoying! Could anyone advise me in what i could do to fix this problem? Also when boiler is off the pressure gauge is at 1.5bar but when its fired up it rises to 2bar is this normal?