Just found this... the forth problem looks promising. Would this cause reverse pressure to be sent back to the reservoir and make it eject fluid out the overflow tube?
13. Overheating Issues
Start by replacing your thermostat and water pump. They are easy wrenching and the parts are cheap. 195 degree thermostat.
Second, check and replace the fan clutch. Chuck the fan back and fourth. Any movement is bad and can cause wear on the water pump bearing. Also, if the fan spins more then a third of the way around cold, it needs replaced.
Third, flush the coolant system. There is a plug at the bottom of the radiator that makes this an easy process. Please don't do this while engine is warm.
Fourth, replace the radiator cap with a new 18 pound cap. The spring can become faulty and cause improper flow. Coolant bubbling in the reservoir is a good way to tell if the cap needs replaced.
Fifth, clean both sides of the radiator. Mud/bug/debris filled fins will not allow proper cooling.
Sixth, burp the cooling system. If there is air in the system it will wreak havoc on the flow. Do this by parking on a steep incline and let the engine run from a cold start for the few minutes with the cap off.
Seventh, for slight overheating at cruise you're looking at a clogged catalytic converter. Low engine vacuum at cruise and high back pressure caused by the clogged exhaust leads to a build-up of heat and pressure in the engine.