water not hot instantly?

hope someone can help, when i turn the hot water tap on on my combi nf80 bolier, the boiler does not fire up, i have to turn it on and off a few times before it fires up and the water then flows hot. Also the pressure falls when i put the heating on. Also the reset buton pops out when the water flows with hot water after around 10-15mins

mike
April 2011