I have had the same problem with my 628 csi 1986 .
I tried to eliminate step by step every possiblity untill
I found out that one of my fuel injectors did not work.
So I was driving on 5 cylinders in stead of six.
I changed the injector and the motor was performing much better then before..
untill I did a kick down to 200 km/h and blew the relating pisting bearings.
My engine did 350 000 km and I used my 628 CSIA on a daily basis.
I have a spare engine,but have not had the courage to put it in.
( text francais sur demande)

Pierre