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Sounds like an intermittent ground connection for the computer, either in the wiring connections Corrosion) or where grounded to engine/frame. Sounds like connection makes sufficient contact due to vibration of car at speed, but with little vibration in wiring connections at slow speed or stopped, ground connection is lost momentarily. You will go broke replacing parts based on guesswork, check the entire computer wiring assembly for corrosion at connectors. The 250,000 miles suggests it could easily be a corroded connection in the wiring harness. Good luck.
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