Sporadic loss of water/airlock in strainer

[pawl]

hi, I'm new here so I hope I'm not butting in at all. However, your problem seems very similar to the problems I had when I first bought my present boat. Every so often, usually when returning from a fairly boisterous sail, the cooling system would fail to pump any sea water. Very annoying as it usually occurred as I wanted to motor to my berth.
My engine is a Yanmar 2GM20, fresh water cooled via a heat exchanger. Like you I checked everything but could not find any good reason why this was happening. To cut a long story short, eventually I fitted a non return valve just after the seacock. This was some years ago and it hasn't happened since, touch wood.

 

[ghostlymoron]

Plenty of o rings on internet. Just specify dia and thickness. Nitrile rubber suitable for this application.
On my boat, the suction runs from the sea cock up to the strainer about 4' then down to the pump. Strainer several inches above sea level so your system should work provided the pumps in good shape, the in/out on the strainer are the right way round and there's a good seal on the lid.