Volvo Penta md11c overheating

[Bill Seal]

I know, I know, Volvo and overheating are sort of synonymous, but mine only does it when it feels like it. Freshwater cooled, it will start overheating at random times, sometimes I can cool it down by speeding up the engine, or slowing it down, or after the engine starts boiling confused: It acts like an air pocket somewhere. I've drilled a small hole in the thermostat, checked out the purge hoses from the fresh water pump and heat exchanger. This weekend I'm going to reverse flush the raw water system, but something confuses me, the raw water goes into the TOP connection on the exchanger. It seems if air got into the water intake this could cause an air pocket to form in the exchanger which wouldn't self-purge into the injection elbow.
Any ideas?