I have an '88 Westy that I was having persistent problems with it
cutting out after driving a few minutes, or after about an hour on the
freeway.
Ultimately I fixed the problem by replacing the ECU.  (I retested the
original, it is burned out )

Thinking the problem solved I was on a ten hour drive to move to a new
job when an hour into the drive the Van started exhibiting the same
symptoms.
A little roadside diagnostics showed the problem.

I had done the capacitor fix as has been outlined in the archives. It
turned out that one of the solder joints broke and one leg of the
capacitor was making intermittent contact.
I removed the capacitor, and installed the factory VW harness fix (Which
I had recently purchased as a just-in-case idea.)  Long story short,
that fixed the problem and the van has run perfectly ever since.

So, if you are having the bucking, stalling problem, and have done the
capacitor fix - make sure you check the solder joints.
My guess is that there is enough vibration so that a weak solder joint
can break free.

Todd
'88 Westy