>>>*H'lo!

I read all this thread, and this does not sound like a misfire.

My best guess since the shudder is midrange and goes away with the air conditioning engaged that there is an out of balance condition. Those tend to show up at around 2500 RPM or so, then again at around 5000 RPM when the condition isn't serious. If WAY out of balance she lets you know right away, just a few grams is tough to chase down.

Engaging the air conditioning increases belt load on front of the engine and that tends to dampen the vibration, something I have seen happen before with inline 4 cylinder engines and some inline sixes, normally following a rebuild when the weights weren't checked...*Shot in the dark but that's what my Dad said, too....:D .*EBjr