Volvo penta 4.3l temperature fluctuating

[cnotecherry]

Kind of makes sense to me now here's my theory the bypass is restricted so the only flow out is through the thermostat not quite enough so that's why it got a little over temp then because the hot coolant from the bypass is not mixing with the colder coolant from the HX a rush of cold coolant goes through engine causing my low temp and closing the tstat and then the cycle continues that would be why it was like clockwork too