shower pump switching off ?

i have a powerforce easiboost shower pump in the airing cupboard next to the hot water cylinder fitted with a surrey flange at the top of the tank the shower valve is a mira 88 which i have just replaced i have also renewed the pipes to the shower pump since replacing the valve the pump turns of after about 3 minutes the water flows through the pump with out the power seeing i replaced the valve would that have any affect on air locks or is it coincidence that the pump as gone wrong you can hear the air coming out when you first turn the shower on and this is the only the only way the air can escape would it pay to fit air valves i have no trouble with the hot or cold water through taps