low shower pressure or no water at all !

[gja01]

Hi i have rs250 Ideal boiler with a immersion heater we don't used, the shower has just been fitted (mixer). It has been fitted with a twin impeller pump, we get a good working shower for approx 2min and then the pressure drops to a dribble, if we then turn it off for approx 5min we get the same again...Could anybody help with this problem ?
Thank you Gavin.