1999 Chevy Venture Wagon is not down-shifting when going up a hill at 70mph. As we drive up hill, in drive, the car begins to slow down because torque is not enough to move the car up the steep grade at same speed. Instead of the car automatically shifting into 3rd gear, it just continues to slow down. If we press down on the gas pedal, this does not induce the downshift properly. This is a new problem that just occurred for the first time about one week ago. The transmission appears to be shifting properly at all other conditions when not at high speed. Any suggestions?