Your problem may be that your power steering pump may be going bad or leaking.Your probably asking yourself what does the power steering pump have to do with the cooling system. Strangely enough Lexus has the power steering pump and your cooling fan connected together. My return hose on my power steering pump was leaking and it was causing the fan to not spin fast enough to cool my car down, therefore the temp gauge would fluctuate, but not overheat. Usually the gauge would rise when I am driving at low speeds or stopped and the gauge would go down when driving at high speeds. I replaced the thermostat, radiator cap, water pump, heads inspected for crack head, and a rebuilt power steering pump. To check for leaks locate your power steering pump and make sure its not leaking. If it is not leaking then the problem is most likely with the hoses. I ended up having to replace my return hose which cost 100 bucks and only Lexus sells it.
Layne
To get rid of that white smoke replace your valve seals and adjust them while your at it.