I have a '09 IS 250 with 130K miles that started overheating intermittently.

First time when the temp rose, it was the radiator cap that was busted. Replaced it, refilled coolant.

Then, changed the thermostat, still the same, erratic temp readings while driving. There is no sign of overheating when I open thee hood. No steam coming out of engine or coolant reservoir.

Afterwards, changed the temp sensor, same behavior.

Lastly, burped the coolant system, may be things got a bit better, but the overhearing still exists. While sitting in traffic, the needle goes towards H and sometimes stays up there, sometimes drops immediately down.

Any thoughts?

Do both of your cooling fans turn on if you have AC turned on? The next time it happens pull over somewhere safe and make sure the fans are running. You could have an intermittent electrical issue.

Sounds like it has air trapped still. Do verify the hose from the fill neck is actually the hose that goes to the reservoir bottom. It is possible to reverse this connection which as you know, when the engine is turned off, it cools off over time and this is when the block pulls a vacuum through the cap, to the pickup tube in the reservoir. If it sucks air, the block will never purge and the result is the problem you face today.

PS they make special funnels just for purging air from this system.

Did you try turning the HEAT to the highest setting when its over heating? It should help.cool it down if there is enough coolant in the system.

Also, do not rely on the reservoir as sanity check of the block being full. Any small leak will prevent the block from sucking coolant from the reservoir.