Also check the 27mm over-pressure switch on the in-take manifold, just in front of the cruise control actuator in this picture here:
http://www.peachparts.com/shopforum/groups/w124-300d-2-5-turbo-picture2068-dsc00027.html
Too much pressure grounds the lead, which then opens the vacuum tube switch to the alda.
If the pressure switch is stuck, i.e. constantly grounding, then your alda will not boost your fuel during turbo time and, ergo, no turbo benefit. Disconnect the lead and check for ground continuity on the switch connector (with the engine off). You should not have any. The switch can get gummed up inside with egr goop. Try cleaning before replacing. Also check that the lead between the over-pressure switch and the vacuum tube switch is not shorting to ground itself.