Again though, heat soak is only when a motor is shut down. Over heating while running is another issue.
http://cjbfire.com/Heatsoak.pdf
Ok, well there really isn't any way for ambient air to get sucked into the cooling system without having coolant leaking somewhere. I have seen a few that there was no outside indication of a leak because it was so small and the coolant was spraying onto the turbos. I am sure you pressure tested the system hot.
If the temps are climbing sitting at idle you may not be getting good circulation. Non-rotary thermostats are common or removal of thermostat.
One other thing I have found on idle overheats is RTV in the system, or Bars leak. See my forum on gaskets and sealants.
Both can restrict coolant flow in the rotary.
Water pump could be a culprit, but I have never seen a Mazda water pump have that kind of failure. I suspect anything that isn't Mazda or I haven't tested on the race cars.
If there is enough air in the system it could hang up in the water pump causing flow issues. You could possible have a lot of air hanging in pump chamber or excess RTV blocking something.
A quality cap is a must. The only other thing I can think of right now might be a hole in the line to the overflow tank.
The way I fill these puppies is
1-set the heater to hot
2-get the nose in the air as high as I can
3-Pull the TB hose
4-fill until fluid comes out the TB hose
4a-on stubborn ones I pour the fluid into the TB hose
5-install TB hose
6-burp by squeezing lower rad hose
7-start car run to temp, let it cool down
8-top off ast if it needs it, usually it doesn’t, occasionally it does.
9-repeat 7 and 8 if needed. There has been a couple that I had to do 7,8 three times.
Another real quick and dirty - Once filled you can put a pressure gauge on the ast, put the 14psi on it. Wait a few minutes remove the gauge, if there is air in the system it will usually migrate to the ast.
I am afraid though from my experience from R100 to RX8 the fine dust bubbles are the first signs of housing seals. To me you are describing all the classic symptoms. Everyone that come into the shop with those symptoms the seals failed within a year. Water pump churning would usually be larger clear bubbles and with the cap off any air in the water pump chamber is expelled quickly. The tiny bubbles will not get larger when revved, until the leak gets worse. Many times they will not over pressurize the system until they get larger. When I have tested these in the early signs I haven't picked up any HC or CO until they got worse.
I tell my clients to buy some time is to drive it easy, relieve the pressure on the system after shut down, and squirrel pennies away. Out here in the desert we may be able to get it to next year before having to do the build.
Not being able to see the car and not knowing it's complete history I am only able to give you my experience. Hopefully in your case I am wrong. Listen I don't like being the bearer I don't want to do a motor unless I really have to or the client wants it. One of the reasons I spent so much time and testing on keeping these things cool.
How many miles on this motor? Original? Water pump or thermo replaced? PM or better yet email me at the shop if you want.