I am betting the boost sensor line from the intake to the overboost switch to the ALDA is clogged with EGR TAR. my 87 300D had the same power problem. EGR tar is a very sticky mixture of oil and soot. some oil, even on a new engine, comes through the crankcase breather which then mixes with soot from the EGR system. take off those lines and clean them... or in my case i just replaced them all (it was easier)
here are the connections of the hoses you should check