When you say downshift, by how many rpms?
0-500 is going to be the TCC locking and unlocking
1,000ish is going to be a true downshift from 4th to 3rd.
TCC will lock and unlock based on load. Unless you are pulling a 7 mile mountain, the behavior is a bit odd and I'd check the switch that Dan mentions.
Actual downshift is about the same, unless you live in the Alps..it'* odd. Temps can be a factor, but really shouldn't. If they are causing it, then something bad is likely beginning to happen inside your trans.
Could also be a TPS sensor that may be giving a wacky reading to the pcm depending on throttle angle. In this case it would be understandable that the pcm is calculating the load etc and the TPS angle to suggest that the lower gear is where you want to be for the best power/etc.
00 Regal GS: All kinds of cool stuff.