I seem to recall you owning a 300-SEL. If I'm wrong, ignore this post.
I have a 300-SEL and had the same problem you described. The first time it occurred, I too suspected air in the system. Reference task 20-010 - Draining, pouring in coolant. This task is on the 126 CD-Rom. Step 7 is very interesting. I've NEVER seen this mentioned on this or any other site. Following the procedure in 20-010 will eliminate trapped air.
The gurgitation in the overflow tank recently reappeared. It turned out that the pressure cap was bad AND that the expansion tank was leaking, thereby not holding pressure. I replaced both along with all hoses.
My cooling system is once again doing fine.
1991 300-SEL - Model 126
M103 - SOHC
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