1999 Pontiac Gran Prix will not shift into fourth gear

1999 Pontiac Gran Prix mileage: 122,000. The car progresses 1-3 gear acceptably. When the ambient temperature is above 40-45F, the car will not shift into fourth gear. Below that temperature fourth will engage appropriately. When driving down a hill in third such that the car is being held back just a bit, you can shift to fourth, the car feels as though fourth engages, but when you begin to accelerate, its obvious the car is still in third gear. Given that the car operates in fourth in cold temperatures, I figure the problem is not mechanical, that it is likely sensor related. I have had the car connected to a computer twice, no codes are stored. Any suggestions?