Thanks for your replies.
I'm using 5W-30 conventional oil. Castrol High Mileage, I think. On a student's budget, I'm not ready to make the jump up to synthetic.
No vacuum leaks, and all of the hoses are in good shape. I'm guessing that one or more of the injectors is dying, and the problem is worst when the air is below freezing for extended periods of time. The injector(s) must drain into the cylinders overnight, and when it's time to start, the DME sends an additional shot of fuel and wets the plugs. Still no codes, even with the rough start.
With 198,000+ miles on what looks like original injectors, I'll just replace them all. It looks like I'll have to take the upper intake manifold off, so I'll need a new upper gasket. Has anyone done this job before?