'overboost' means the MAP
sensor has detected boost pressure exceeding a setpoint;
at which point the ECM
shuts off fuel to the engine,
resulting in "Overboost Fuel Cutout" .
sensor shows boost P below setpoint, ECM
turns fuel back on.
this can result in a cycling:
overboost surge / fuel cutout / overboost surge / fuel cutout ...
if you keep your foot in it
on an older un used TD
9/10 of the time its a plastic vacuum line, elbow, or connector thats leaking.
our old TDs used a lot of plastic vacuum lines which become brittle , crack, and leak.
this can be a Nightmare for driveability,
and can cause all sorts of random operating issues.
I suggest a THOROUGH inspection of all vacuum lines, connectors, etc;
replace any/all suspect...
unless you want maintain it as stock appearance,
I further suggest replacing all original vac lines with new vac line,
auto stores & hardware stores carry it by the foot in numerous inner diameters,
aftermarket also offer 'silicone' vac lines if yu prefer (or want a color)