engine or trans problem

[loki]

my 99 M.C. w/ the 3.1L (now w/ about 130K) started this problem last winter....after the car has warmed-up to the normal temp it gets to on the temp gauge, it can sometimes unlock (or not lock in in the first place) the tourque converter in overdrive....sometimes in 3rd too. going 55mph i can rev the engine between about 1500rmp to 2500rpm w/o an upshift or a big change in throtle..... kinda like if it was almost in nuetral but still moving the car. would this be the same thing as if going 55 also, and sometimes the engine drops to near idle speed instead of having engine drag closer to ~ 1400rpm. these things seem to only happen in cool and cold weather, hence it's starting to happen again since fall is almost here. any good ideas are very welcome , and thanks in advance. and would this cause the engine to also try to die at idle (but it does correct itself) once in a while(no more than 1 time a day, usually)?