2005 E320 Sedan - white smoke from exhaust?

The car is a 2005 E320 sedan gas engine, my wife was driving in town not on the freeway and felt a flutter in the engine with a slight loss of compression and a poof of white smoke from the exhaust. Seconds later the smoke dissapeared and the car began to run normal again, no noises no power loss. I took the car to a mercedes technitian, he changed the oil and filter ran a detergent through the motor and stated the oil look very clumpy and sluggish and evidence of oil on one of the spark plugs. No sign of fluid in the oil or on the filler cap. The car then ran fine for the first 30 minuets until it warmed up. At low rpms approx 10-30mph the car will flutter and loose temporary commpression and a large cloud of white smoke will appear out of the exhaust pipe, then it runs fine w/no power loss for a 15-20 minutes then another poof of white smoke and loss of compression then returns to normal again. In between the poofs of white smoke (20 minutes) the exhause runs clean no smoke at all. The smoke is not steam it is smoke that stays in the air for a while. The mechanic seems to think it may be dirt particles preventing the valves from shutting properly allowing oil to seep into the cylnders. First thing in the morning, no smoke al all, after driving the car approx 20 minutes the first poof of smoke will appear. The car has never over heated, the fluid levels are normal and oil level is normal. Help!