I've been told that you spray water at the intake manifold gasket to find a leak. I suppose you can see the water bubble at the point of the leakage, or perhaps the engine will run poorly for a moment as the water is ingested into the intake manifold.
I get the feeling that it's not too common.
In your situation, I'd suggest it's your throttle actuator. On M104 engines like ours, the cruise control function is integrated into the throttle actuator, and a failing / intermittent / mal-operating cruise control is the first sign that the actuator is on its way out.
Yeah, it sure is amazing how characteristic problems arise at similar mileage on these engines...
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".