Potterton EP 2000 - intermittent water?

We are having intermittent water issues. At various times during the day their will be no water in the upstairs bathroom and the only very weak water downstairs. In the mornings it will often occur between 6:30-9:00am and around early afternoon and in the evenings between 8:30-10:30. Sometimes I can get the pump working again by turning it off and on, but not always. Does anyone have any ideas?