x3 on the Bango bolt and line... Had that issue on all 6 of the w123's I have owned. Never hurts to get a cheap boost gauge and see exactly what boost your actually getting, simple to install, 5 minutes tops.

Lift pump, I had that issue on my current w123, again, simple fix.

Check that most over looked tank filter.

Run a couple cans of Diesel purge through the system, it actually works wonders...