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trans problem
based on all your information this problem is almost certainly not mechanical but hydraulic.i suspect that the command valve for 3rd gear is sticking in the valve body and by running up to red line and then backing off on the throttle you drop down the pressure that is holding 2nd gear engaged which allows govenor pressure to overcome it and push home the command piston in the valve body for 3rd gear.this is not a diy job you will need the help of a skilled pro unless you elect to just buy a rebuilt valve body from mercedes and put it in.all your other gears work correctly except for third [and only then under full throttle] probably caused by minor defect in piston bore in valve body aggravated by very little use and lots of sitting around.
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