My first winter with my '92 e34. Had similar experience once it got cold with revs going up and down, tho not quite as much as yours. Also check engine light coming on occasionally. After coming back from a trip on the interstate (40 miles each way), it's staying at around 1200 and no check engine light. Seems to need to get heated up a bit to clean things out and settle it down. -Rich
PS: if that doesn't work, tell your wife you'll miss her.