How can i fix my 38cxi ultracom?

Has any one had problems with their cxi ultracom hot water not staying hot when trying to run bath or shower high flow but at low flow at kitchen sink boiler can cope ok (modulates up and down to hold temp) when trying to run bath or shower starts hot for 20 secs then goes cold (the boiler has digital display which gives full flame then drops to only one flame on display) Gas board have changed plate to plate heat exchanger and thermistors still the same fault cetral heating working fine (system been flushed twice in past ) any help would be very grateful.