1994 Skylark Transmission

This one's driving me crazy. Wife has '94 Skylark Custom, automatic, 3.1L engine, 105K miles. Warm weather the tranny shifts perfectly, but once it gets below freezing shift into reverse and it doesn't move, same with Drive. Sometimes if she shifts from Drive into 1st, off it goes and it as she shifts back into Drive, all is well. Same with reverse, shift from Drive into 1st and then into Reverse and off it goes. Return to Drive and it acts as though all is well in the world. Any assistance or ideas would be greatly appreciated.
Thanks!