1997 Mercury Sable Delayed Shift Between 1st and 2nd

I have a 97 Sable LS Wagon w/ a 3.0 24v engine and console shift. There are instances when it won't shift from 1st to 2nd unless I wind it up to 3500 rpm and let off the gas completely. I've noticed that this only happens when I've been driving at highway speeds for about 20 minutes or more and I then start driving in the city. It's more pronounced if I drive at 70 or above so I keep the cruise at around 67 mph. I've read about shift solenoids, VSS and other such maladies but my speedo works fine and so does my cruise control. I would hate to put two grand into a new tranny only to find it's not the problem. Help!!!