1996 E300 Diesel Surging. Please Help!

[rthomas63]

I own a Mercedes Diesel E300 with 48,000 miles. 3 months ago the car began surging and lunging between 2000 and 2500 rpm (between the speeds of 55 - 68). If you are going over 68 mph it is fine. It does not do this all of the time but it does it more often than not at all. It has been taken to a diesel mechanic who specializes in fuel injectors and he said they were fine. Filters were changed in November. Took it to the local Mercedes dealership where they replaced vacuum sensor and another part ($500 + ). This did not solve the problem. Does anyone know what the problem is? I would appreciate any help or suggestions.