Bad grounds can play havock on a Vanagon engine.  I would check the ground
strap on the driver's side head for corrosion and security of attachment.
You can buy a nice new insulated one (I like the insulation for corrosion
resistance) from Wal-mart for a couple of dollars (Exide near the battery
section).
Make sure the connections are clean, etc.
When you turn the van off and restart it you are resetting the computer.
Could also either be bad temp sender, AFM, Oxygen sensor, or ECU itself.
Check the grounds first though and then check the distributor cap and rotor
(cheapest and easiest to check and fix.)
Hope this helps.
Ken Wilford
Van-Again
John 3:16