Trouble when raining is sometimes the O2 sensor, or
the wiring from it, getting wet or cooling down.

A test is to unplug the O2 sensor.  Then the van
runs in "default mode" which is a little rich, but
should not have the jerky problems.

Ken Lewis's little gizmo is what you need.  It is fun
to watch the dancing LEDs while bored while floored--
and it tells you about your O2 sensor, too!  ;-)
http://neksiwel.20m.com/photo5.html
Also, lots of good info about the O2 sensor.

Hope this helps!

Richard
Boulder