Most likely a fuel issue, either your injection pump is not getting full stroke (linkage issue) or you've got a semi-plugged fuel filter. Could also be plugged air filter, or plugged exhaust.

If your engine also produces too much smoke, and the exhaust smells of raw diesel, you may have an EGR failed in the open position. Those were the symptoms on my 123.190 (300TD) which was a non-turbo car. Perhaps the turbo charger would mask some of those symptoms?

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC