Unknown P0302 Tech at shop has no clue.

01 6.0 in a Yukon Xl. Cyl #2 misfiring. Parts changed/checked ; Head Gaskets, with valve job. Coil. Plug wire. Plug. Injector. Fuel psi reg. I had a p0 or o302 code, it took the shop about a week to figure out that I had a valve not closing. They did a valve job, that took more than a week. Right at 100 mi the ses light came back on. I took it back and they still are not able to pinpoint this problem. They are a good shop that has been operating for over 40 years now, so I don't think it is a shady shop. But they are sending it to a dealer now due to being so stumped. Do you guys have any ideas? I am having them replace the fuel filter Monday when I take it back, But just any info that may help them out would be great. P.S. The miss is a soft one, only felt at idle, in gear or in park.