Yeah, sure sounds like an ETM. I had the same thing happen to my 2001 XC, after a week of surging like you described. I had it into the shop before a big trip to the mountains, but like you they couldn't find any codes. I hadn't had the "performance reduced" message yet though.

As you would expect, the thing totally conked out pulling up a long hill out of the Sierras. "Performance Reduced" message, the thing would barely idle in neutral. Being a SW engineer, I "rebooted" the car, and the problem went away for another hour of driving. I made it home in that way, and the dealer was able to pull a code the next day.

I think your ETM has an extended warranty to 70k miles, since they know they had serious problems in that area. If you make enough fuss, they should change it for you, or at least install a "black box" to record what's happening while you're on the road.