My alita 40 has taken to stopping and starting intermittently. The troubleshooting section of the manual suggests overheating. But it is outside in fresh air. I have cut down from 3 to 1 stone and it still keeps happening. Does anyone recognise this problem; also my pond is 1200 gallons -7ft diameter and 4 foot deep. I am worried about the oxygen levels while I find a fix. How do i know if the oxygen levels are too low and causing problems?