Perhaps the O2 sensor is shot? I'm not sure about the E420 - but in other cars the O2 sensor doesn't activate until the exhaust warms it up. Perhaps it's not doing its job well and sending an erroneous signal to the engine after it warms up (your 10 minute time period) - causing it to run lean or rich.

No idea why it would occur after sitting for several days.

Any thoughts from others?

Troy
1995 E420