Hello and welcome!
Thank you for requesting me!
Based on my experience, the engine will misfire only at idle either because there is an air or a vacuum leak near that cylinder (intake manifold gasket leak) and will cause the air fuel ratio to be off enough at idle (way too lean) causing a misfire at idle or this is because an engine mechanical issue.
Every once in a while a bad fuel injector
or computer/wiring issue would cause the fuel injector to stay wide open at idle causing that cylinder to misfire. However, this is very rare on this vehicle. First thing is first, I would have checked the intake manifold gaskets for leaks before pulling things apart. Chances are that this was done provided there was a good tech working on the vehicle. If this was not an issue, then chances are it is going to be an engine mechanical issue going on causing this misfire condition to occur.
If it was an engine mechanical condition causing this to happen, then this could have been seen on a hand held engine vacuum gauge. At idle, it should be about 19 to 21 inches or solid steady engine vacuum. If the vacuum gauge needle was fluctuating at idle, this would indicate a mechanical issue going on with the engine. Valves, lifters seats or even camshaft issue. If this was what was found by the technician when the engine was at idle, then yes, they are on the right track. Usually on these motors the valves don't seat correctly causing the engine to misfire so this needs to be checked while the cylinders heads are off as well as the camshaft. Once things are apart, the tech should be able to see any wear on the valves, heads or camshaft that could cause the engine to misfire like you are describing.
I hope this helps!