If the reg was leaking, you would see gas in the vac line after a short run/idle. [ 2-3 min] It is possible not to have gas coming out of the reg port with the vac hose off and engine running , but still have a leak when the hose is on b/c with hose on , you have fuel pressure on one side of the Reg diaphragm plus engine vac sucking on the other side...meaning they are more likely to leak-by when they are hooked up then when the hose is off.
So, the best of the two test is to look for gas in the line after a 2-3 min ilde..that will be max condition for possible leak-by.
If NO, that is not your problem......but that does not eliminate a possible vac leak .
One causes rich adaptation and the other causes lean adaptations.
The first Possible Remedies list by Aldata for limit conditions are " Vac Leak/FP Regulator"..so, they should both be checked...what one is looking for is Unmetered air or fuel...which the ECU can not handle.

But then again, that ECU does not have a good lee-way..so, the question remains..How bad is adaptation requirements and why..
Benz made a change , so a change may be in order...just don't want to do that on assumption, that's all. Specially when FP reg and vac leaks are known common causes for that exact condition.

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A Dalton