Chevy & GMC Vans

I just had the intake manifold gasket replaced on my 2003 Astro. It was running fine after the repair. A week and a half later I took a 1.5hr drive and engine started shuddering and engine light came on. Pulled over an put the scanner to it and got a P0305 code. Cylinder 5 misfire. It stayed like that for a couple of days. Yesterday drove it around the city with no issues and the engine light turned off. Today went to work on the highway with no issues. Wondering if driving for long periods is causing an issue. I am bringing it back to the shop that fixed the manifold gasket. Any ideas why I get misfire only when driving long distance and not when driving normally in the city?

Hi,
I too have a 2003AWD Astro with 120,000 miles, 3.42 axles, 2 inch lift kit , roof rack, heavy front bumper and grille guard,
Sometime with a full load of camping gear driving fast up steep hills from Bakersfield to Ridgecrest I would get a temporary misfire check engine code. It would clear with easier driving.
AP

Maybe bad gasoline, or I should have down shifted on hill?