problem with hot water on ariston boiler?

hi all
i have ariston boiler which we inherited when moving in to house but have major problem with hot water.
when running a bath the tap has to be open slightly for the water to be hot if opened up more it goes cold also the boiler flame always cuts in and out when running bath, the heating comes on even though its switched off and its just getting beyond a joke now it takes an hour or more to run a bath by the time the water goes hot then cold then hot then cold etc.
could anyone tell me what the hell is wrong with it before i pull it off the wall.
many thanks in advance
Glen