I'm not sure how long you've owned your car or when you started paying attention to the oil pressure gauge, but what you've just described the oil pressure gauge functioning properly.
The surging idle is a different problem altogether, and is usually the result of a vacuum leak. The oil pressure gauge just responds in kind to what the engine is doing. Mine has the surge problem, and the needle bounces along.
Unlike some other makes where the gauge needles just leap to some stasis point and remain there, MB gauges actually "tell" you something.
Here is what you should see (on average).
When car is cold (and oil is cold and thick), pressure gauge should reach "2" or above.
After warming up, it will go down to as low as "1" on idle.
Pressure reads between "2" and "3" during easy cruising and pegs to "3" during acceleration.
If you have a REAL oil pressure problem, your readings would be "1" or lower ALL of the time.
BTW, the measurement is in bar...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car