@zip439, thanks for the reply. To answer you comments in order...

The plugs were changed ~20K ago by the prior shop. I’ve not pulled them yet, so can’t verify gap or brand/type.

The routing of the spark plug wires is correct. Given that it gives different error codes each time, I doubt that it is a plug or wire issue.

Regarding the heat, the misfire was so pronounced on hot days, it was undrivable, regardless of throttle position. However, the WOT throttle misfire happens at any temp.

This is an SRT6, so no MAF.

The coil packs were replaced with used Bosch units off a running engine.

I don’t have the means to check the fuel pressure, so when I’m down to that I’ll find another shop to take it to and have a pro look at it again. That said, since changing the pump and filter there has been no change in the WOT issue, so while it’s still a possibility that fuel pressure could be an issue, it seems unlikely. Stranger things have happened, I know.

It could be exhaust-related. At 140K, I could imagine the cats being restricted. Is there any way to check that in a home shop?

I’ve already gone down the route of a ‘pro’ Mercedes shop looking at it and have lost all confidence in them. Any shop that tells me they can’t do anything unless I can tell them the codes isn’t worth $10/hr, let alone the $110/hr. they charge.