98 mercury mystique shifting wrong only when cold out

Ok here is the runaround:
98 mystique v6. 133,575mi and 20k miles on rebuilt tranny 2 yrs ago.

only when it is cold out (in the 30's), it will shift hard between 1-2 and thwen the rpms will stick and stay around 2k until you stop and then the tranny will downshift to 1 and then everything is ok. then once the car warms up, and you shut it off and start it, everything is fine and the car shifts like a dream. I changed the fluid and not a lick of difference. any ideas, bad sensor maybe??? please help.

Scott