Sure sounds like a speed sensor or speed sensor wire/connector, Art.
Check out this thread on the 300M ezboard for location of the speed
sensors and their connectors:
http://p209.ezboard.com/f300menthusiastsclubfrm23.showMessage?topicID=46.topic

Since speedo is working, it would be the input sensor.  Inspect the
connector for trash/corrosion, clean it, and pack it with some good
electrical grade silicone grease.

I've seen some indications on forums that the sensors will act up in
damp weather (which maybe points to the connector?), so the fact that it
sat for a few weeks unused may (or may not) explain the strange timing
of the failure without any culpability of the dealer.

For $20 and 20 minutes of your time, it would be nice to find out that
that's it (or at least rule it out).  Of course in theory you should
take it back to the dealer and let them diagnose it, but you don't know
how succesful that will be, or if you can convicne them that they should
pay for it, and for them to just replace the sensor, if you pay for it,
will cost a lot more than $20 (then there's "diagnostic" time, etc.).

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
adddress with the letter 'x')