This is a guess...but I had a similar problem with my 6.5ltr. TD Blazer when the EGR valve would get full of carbon and slugged up. I use to clean the EGR
valve and port periodically with a little carb cleaner and a popcicle stick to scoop out the thick stuff. The EGR on your MB is obviously different but just pull it and check to make sure its clean and functioning properly.
Did these "professionals" check fuel delivery pressure from the injection pump?
Or did they just replace lines?
Pull the fuel filter (if you have not already done it) and fill it or a new one with Diesel Kleen and put it back on while the engine is cold and run it. See if that helps at all when it warms up. I know its frustrating but try the cheapist things first....
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 FRED
Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K
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