Backpressure
Maybe this will help. Check the hose from the exchanger to the vent loop beneath the counter for soft spots. Mine had and electrical tie used for support and the tie was tight and over the years weakened the hose, When the engine shuts off, water drained back towards the pump collapsing the hose. When restarting, water pressure crimped the hose at the tie. At least that was my problem. Before I found the problem I had the pump apart twice, the water filter off twice and then I started the engine with the hose from the pump off at the exchanger and I had good flow, so the problem was further up. Doubt if it is back pressure from the engine. Good luck.
