I had a similar problem with my 18 ,it would run in the garage fine but would cut out 1/2 a mile down the road .I'd push it back strip it down but was unable to find anything then a couple of hours later it would start first kick and the game would start again. Anyway what it was, was a frayed cable at the pick up at the rotor which when it got warm and with a bit of vibration would short out and the PGM would kind of shut down. This was found after replacing the PGM and most of the multipin plugs with bullet connectors. It may be worth a good look.