I have a Pondmaster Magdrive pump (1800 gph) that is acting funny; It will run for about an hour then just stop (not throwing a breaker, it just stops working). I can unplug it for about 20 minutes, plug back in and it works for about an hour then stops again. It is fairly old and I have replaced the impeller a couple of times but current inpeller is only 3 months old. I took it apart and did not find and restrictions and the impeller is in good shape. Any suggestion / testing methods will be greatly appreciated.