Periodically, the transmission won't shift out of first gear on my 98 Dakota. If I stop and switch to Park and then back to Drive, it works fine and shifts normally. I've noticed this only happens after I back up and then shift to Drive from Reverse. The tranny has fairly regular maintenance and has no other issues. This doesn't happen all the time; only after running at normal temps for a while and still being hot. Any help/advice appreciated..