2002 Volkswagen Jetta Cold starts in cold weather

Engine Performance problem
2002 Volkswagen Jetta 4 cyl Front Wheel Drive Manual 127K miles

I have an '02 Jetta 1.8T AWP, manual, 127K mi, that has just started its second winter in Denver. Until then, it was in Florida and never experienced cold. This winter, I'm having a problem I didn't have the first winter. Cold starts in cold weather require a minute or two to warm up before the car runs right. It has super-low power, barely enough to move the car, almost dying, then suddenly catching and leaping forward before almost dying again. Appears to me to be running too lean for the cold. Once it warms up, it runs perfectly. I would have thought it was maybe the O2 sensor, but it's not giving a CEL. Any tips on where to look would be most appreciated. Thanks!