1999 Plymouth Neon Cascade of Problems

Started driving Sunday when speedometer cut out. Tac temp and gas gauges were ok. Speedometer cut back in and out, then gas warning went off (expected) took foot off gas and pulled into gas station, check engine light came on. Played codes through engine light, 12, 72, 42, 55. Disconnected battery for 5 minutes, reconnected and allowed car to rest overnight. Monday morning, started car, in addition to speedometer, radio did not work. Hit a bump, radio came on, speedometer started working intermittenly. After roughly 50 miles, all guages and lights stopped working. Started tapping on dash to right of display, over the radio. Dash display reinitallized, (Air Bag light for 3 seconds, Check Engine on for a second etc. And continued working, however Tach was stuck at about 2500RPM. Drove car another 50 miles, parked 8 hours and returned home. At the end of the 100 miles, pulled into parking lot like previous nite and check engine came on. Same four codes. Also on return, ocassionally engine would seem to increase speed, or work harder never registering more than two gage needles width change in Tach or Speedometer. Where do I Start?