Here's one that's got me stumped on my 79 bus:

Someone suggested this might be a vacuum problem, though darnit if I can
find a leak. The idle is too high when it's warmed up (not before, it
tends to be too low then) and when I hook up the tach meter and try to
adjust it down to 950RPM, it falls down to around 650--700.  I try to
adjust it up to 950, it flys up to around 1300.  It won't stay at 950.

The air flow meter is new (not rebuilt), the oxygen sensor is new, the
head temperature sensor is new.  Got me a Probst book on the way to help
me figure out the Bosch fuel injection system, but would appreciate any
insight anyone's got into the thing.

Erik