When the engine first starts it is in a 'cold start mode' where the
computer defaults to a predetermined condition and ignores the sensors.
A good way to check to see if it is sensors, which is what it sounds
like to me, it when the engine starts acting up turn of the ignition and
immediately restart.  This should put you back in cold start for a short
time.  I had a similar problem when the engine would die at idle and run
like crap after running for apx. 2 to 3 min.  I replaced the No. 2
"blue" temp sender and fixed the problem.  This sensor is prone to
failure and is cheap, I got mine for $10.  It is located on the side of
the thermostat housing (on the 2.1's).

dave
87 Vanagon, 88 Fox
www.251.org