Hi Dave,
I'm still chasing the culprit on my V70R AWD. My cars idle has started to deteriorate as well. I finally got the check engine light to come on, and got a 303 code, missfire on #3 clylinder. I switched the coil packs around, but I'm still getting a 303 DTC. I don't think the ECM is really able to discern what the problem is at this point.
The car won't really run above 3000RPM, and only at part throttle. I'm at the point where I'm going to drive it for a while longer, until maybe I get an ETS light or some other DTC. I'm hesitant to take it to the shop just yet, and have them go on a $500 fishing expedition.

The seemingly fluctuating voltage is interesting. I've been to the NHTSA website and read a few notes about the B+ cable to the main fuse box having an issue. I don't know if it applies to your model or not. Just a thought. There's a few posts here as well.
I think the 70,000 mile emissions warranty is an uphill battle at best. If you look at your warranty pamphlet, there's a section on it, but they specify which parts are covered, like exhaust manifolds and catylytic converters. Believe it or not, my car passed smog with the stumbling/missing problem.

I'm going to go over the wiring harness one more time and see if I can find anything. I'm curious what your dealer says, especially if there aren't any DTCs stored.

Good luck, Mark