Hello and welcome.
Another possibility is a head gasket problem.
This engine in know for the rear headgasket to blow and cause overheating. A blown gasket can cause combustion gases to be forced into the cooling system causing air pockets. Air pockets will cause the roblems you are having.
Suggest a a cooling system pressure test.
Removing spark plugs and inspecting for signs of burning coolant would show this problem.
Ya, nobody likes to hear that, but it is quite probable.