It definately sound like you have a flow problem or possibly the heads still have a problem or an internal crack pressurising the cooling system. Did you have the heads checked for cracks or only resurfacing? I am including as much information as I can find on this vehicle for you. I hope this will be helpful. You also said you do not have a thermostat, the listing I have shows that it does use a thermostat. Also there is a tester that checks for head gasket leaks by sniffing for combustion gases. It is shown on the left side picture(the long tube with the squeeze handle) it has blue fluid in it that changes color when exposed to combustion gases and comes with good instructions. This might help determine if it is still a head problem. If the cooling system is becoming pressurized you will loose flow due to combustion pressures.I purchased on from MATCO and it is called a cylinder head leak detector. Also does the additonal fan come on. I am not sure if the vehicle is equipted with this but it is in the repair manual so I am including it. We don't see too many of these in the shop anymore but I will try to help you as best as I can.Thank you. I hope this will be helpful in some way.
BMW Mechanic
Level 1 BMW Master Certification, 9 Years Dealer experience, Team Leader, Shop Foreman