Shower Pump Cutting Out

We have a Grundfos hot and cold feed shower pump that cuts out after about 12 minutes. The cold water tank is about a third full and there is still hot water in the hot water tank. Warm water still comes out of the shower but obviously not very fast. After a couple of hours the pump then starts working again. All the filters have been cleaned but still not made a difference. Don't really want to replace the pump until I know what the problem is. Thanks