Idle speed miss on #5 cylinder

1997 Dodge Ram 1500 4x4 automatic 350 ci, 201,000 miles.
While on long trip, developed a "miss" when stopped or at idle, not noticed when traveling, and CEL came on. I took it in to good mechanic, his computer showed a miss on #5 cylinder, but only at idle. Cleaned injectors, checked all plug wires, spark plugs, exchanged with others, replaced distributor cap and rotor with new one, still missing. Replaced intake manifold gasket after finding similar problem on web. After all that, still missing on #5 cylinder, but only at idle as before. CEL still comes on after being turned off. He's stumped, so am I.