Ambient temperatures have an affect on fluids and pressures. I believe that the colder the ambient temp is, the more viscous the fluid is, hence lowering line pressures. but then again, Im not sure if that relates to the problem your having. hopefully a gearhead can confirm that.
Temp could definitely be the issue. I'm from Minnesota and during the winter i get a harsher 1-2 shift (nothing to extreme). Been doing for most of the winter (its friggen cold here). Recently the temp made it to 52F and it worked fine.
I seem to have better luck with Mobil 1 ATF for helping to remedy some of those types of issues.
It'* viscosity stays more consistent. It doesn't seem to get so thick when it gets cold and doesn't thin out so much when it gets hot. - 2 thumbs!!!
I have the same problem on my 01 bonneville with 170000 on it,it will shift fine for days then act up,I did notice the fluid level was alittle above the full mark after driving it for a long time,will take some out soon.
Try checking the long-shift adapter.. I had the same proplem, took it to the dealer and they had to replace the long-shift adapter. They had to take out the tranny to do it but the car (2001 sle) shifts smooth as butter now.