Shower pump very noisy?

had a new bath fitted and because of low pressure the plumber fitter a bristan shower pump to pump only the hot water to the bath. it is a twin impeller but he only connected the hot side and the cold is to the mains. the chap at the plumbers merchants said it was fine to just connect one side. it was ok the 1st day after the job was done. now on day 3 onwards when i turn the bath on sometimes the pump works fine a quiet hum but then woosh it goes in to overdrive like a hoover on steroids ! as it does this the hot water from the tap slows right down , if i turn the tap on and off a few times it seems to correct itself, any ideas welcomed, i did wonder if as i read somewhere the cold should also go through the pump maybe to balance it or lubricate it,