This is a 2001 Custom Explorer van. The problem existed from the day of purchase. Dealer never found the problem. No longer under warrenty.

At sudden acceleration speeds generally over 60 mph or riding over Blue Ridge Mountains, service engine light comes on and van starts to shudder. I take my foot off the gas and reduce speed and the shuddering stops and the light eventually goes out. Van runs well on flat roads without taxing acceleration. Any ideas as to what is causing the problem?