fluctuating hot and cold water

Dear Mike,

Hi, unfortunately you are too far away from me to be able to take a look at my pulsacoil boiler. i just recently moved into this property which already had the pulsacoil a-class boiler in it. it seems to work fine except the water even though hot fluctuates hot to warm to cold to warm to hot all the time in all the taps in the flat..more annoying and obvious in the shower of course. i have had many plumbers in to look at it one of which tried changing the sensors...which did nothing. he then spoke with gledhill technical support, who initially advised the problem to be the sensors but as that didnt solve the problem then did a full error check and advised the problem to be the PC board. the plumber then ordered this PCB and fitted it however this still didnt resolve the fluctuations either! i would really like to know where to go next as i have no clue nor does the plumber as to the problem. what i have realised is that when you turn the water on, the pump kicks in and i can hear it but when this is on, the water is cold and then the pump stops and there's no pump noise and the water gets hotter, then again the pump kicks in and the water goes cold...and so on. please please please help me!

thank you,
Yayganeh (London)