Thank you for your replies.
1- The oil is clear like new (Just got the 60,000 miles service)
2- The coolant is definitely not overfilled. In fact, last night, I removed the cap, started the engine with a low coolant level (Bottom of the expansion tank), and then suddenly, the coolant went up and spilled profusely all over.
What's puzzling me is that it's intermittent. I drove the car 20 miles after replacing the thermostat, with no problems (Just lost two cups of collant). Then I got this problem again yesterday after driving two miles, and the temperature was a normal 185F. It looks like suddenly some gases escape into the coolant and push up the coolant out of the expansion tank.
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