Still Water Pump issues

[RonS]

I posted this under plumbing above but will also try to get answers here. I have the on demand type pump 5900 by Surflow and it no longer pumps up to full speed/pressure. Surges and goes down to nothing for a few seconds, other times will not even kick in when water is being used. This has been ongoing issue, and pump has been replaced twice, and then only worked properly for a short time. Something is not right. I have replaced all the filters, checked all the water lines as far as I can reach. I'm thinking maybe something in the fresh water tank or water line plugging it up and not allowing pump to operate?? Maybe when the holes for the various hoses/connectors etc that the plug or drill fillings fell into the tank? Any ideas of what to try to make pump operate, hard to take a shower using the pump, so always try to be on city water.