2004 Subaru Forester Antifreeze overflows, gurgles, then eq

Engine Cooling problem
2004 Subaru Forester 4 cyl All Wheel Drive 75000 miles
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This issues started last week. I noticed anitfreeze odor and spill on the garage floor after driving. Reservoir tank was completely filled, no a/f in the upper hose to the radiator. After the car sat for a couple minutes, I heard gurgling and the a/f drained from the overflow and I could feel a/f in the hose.

I took it to my trusted mechanic and we thought stuck thermostat. It was replaced along with a/f. A week latter same thing happen, overflowed, reservoir tank completely full (and bubbling from heat), no a/f in the upper hose, sits then equilizes. This happened twice in one day and I noticed that it seemed to occured if the car did not have a chance to cool a little, so it occured after continous dirving/mulitple short stops.

Thermostat seems working probably as the temp guage goes up then settles back, running mostly where it always does/did.

Suggestions appreciated.