My 2004 Forester XT auto, 96,000 miles started missing, stumbled, and stopped going uphill on the grapevine upgrade on the way back to Los Angeles. I was nearly hits by trucks as I slowed to the side. Car restarted and limped home. Shop analysis led to the replacement of the spark plugs. The car once again missed and stumbled but did not stop on the upgrade back to LA from Ventura. I must regularly use the grapevine and need help to solve this problem.