I would put the title to my car that it is the converter not locking up. I know from experience. And whoever thinks it can't raise a couple thousand RPM's has never had it happen to them! Mine was in the tune. Something about the converter and speed, along with the tire size I had. Yours is probably just not warmed up. But mine would be fine during cruising on the the highway, and then all of a sudden the converter would unlock and the rpm's would jump up like it was in 3rd. I drove like this for about 800 miles. I would have to pull over, turn the truck off, start it back up, and it would be fine for about 30 miles then it would start doin it again. I had to do this, as I was getting my wife to her mom's house after her father was killed in an accident. If it had not been for that I would have just turned around and came home. That trip ended up burning up the lock up clutches. It got to where it would never lock up. Ended up gettin the tranny rebuilt, and another tune, and it was fine. Your's might not be the tune, it could be the cold weather, but I would def. look at the tune also.