Mercedes: 220cdi loss of acceleration, exhaust system, acceleration

Question
2003 model, loses acceleration for no reason. Can be okay and then it's gone. Can zip up a hill and then next time it can just say make it up to the top.

Answer
John,
I need additional symptoms.. Is it misfiring. Or is the transmission causting the engine to over speed.
Is there any bad odor coming from the exhaust system.
When is the last time the filters were changed. When was the last time the plug were changed?
How many miles on this vehicle.
Thanks
Rob