This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
The problem is the flywheel sensor. It is located by following the
wires from the along the firewall and along the bellhousing. These
sensors get worn out from debris and it also might be just the
wires going to it. I had the same problem and took it to a dealer
and they couldn't figure it out either. (Ken Talley)