It would seem that a trip to a knowledgeable dealer might
be a good idea to consider, giving them advance warning
and description of the symptoms, of course.
So, write up a page to fax to the service department, with
a clear history of your car's symptoms, then call some dealers,
talk to the service department, and try to determine if
they have the skills, and willingness, to investigate the
situation.
When you take the car in, be sure to demonstrate the
"condition" to them, and have them drive and reproduce
the same symptoms, twice or thrice. You do not want
the car to spend several days there, only to get the
response, "we couldn't reproduce the symptoms".