Yeah, I just started noticing it since it's gotten colder out. Didn't notice it at all last winter, but then again, I didn't drive it a bunch last winter either. I have been driving it for the past 3 weeks as I procrastinate on getting my door weather deflectors resealed on my Bonneville.

 

I have seen this to be more common in the electronic controled transmissions.

If the history of the torque converter serves me right, it has to do with the TCC (torque converter clutch) controler.

Its a GM thing. SUPPER common with the mini vans, lumina, corsica, ect....

Some are corrected by doing fluid and filter changes, most need that TCC changed out (some who don't, end up changing out the trany) due to the torque converter takes out the trany.

On most 95's and up also check your shifter cable (could be signs of the cable has too much play). Not fully engaging in park. And there is no adjusting the cable, it's either right or wrong (replacement only).

EDIT:

Oh and I am surprised that the mechanics here (there are a few ASE Cert. ones) have not posted this due to it is a very common issue with GM's.