After about 15 min on the highway, the "Service Engine" light comes on. It stays on as long as I'm driving then shuts off when I stop. Upon restarting it stays off as long as I'm not driving at highway speeds. A new wrinkle is, when driving at highway speed, after the light comes on, at a seemingly random point, it feels like the trans downshifts for a second, engine rpm increases, then it upshifts again and rpm drops back to normal for the speed, and the "Service Engine" light goes off. This all happens very quickly, just a second or two. I have experienced the "Service Engine" light returning again after another 15 or so minutes, but not the downshift in the same trip. All trips where this has occurred have been less than an hour in length and I haven't had cause to make a longer trip since the shift issue started, so I haven't seen the combination of symptoms over a longer period yet. Yesterday I made a half-hour trip where the trans did the down-up shift on the way out, but not on the way back, same route, similar speeds each way. I'm guessing that the light and shift are related to torque converter lockup since that doesn't occur until over 40mph (about 42-43 by my observation), but the last time I had it checked no codes came up in the diagnostic. Any insights out there?