Elsie,
Strange problem. If it were just bearing or seal noise I would expect the pressure would be up immediately after starting as the shaft and impeller are turning at the right speed. If a bearing is seizing enough to keep the RPMs down and not pump water, 0 pressure, then in the 5-15 minute time frame there would be enough heat generated to burn up the bearing totally. Are you sure you have a full water charge? i.e. is the pump basket totally filled when the problem is present? Are there air bubbles out the return jets? Sure sounds like you have trapped air or air leaking into the suction side somewhere and it just takes that time to blow out.
Best I can think of for now.
Al