Automatic Transmission problems of the 2003 Lexus GX470

When descending a hill at 40-45 mph in drive and gently pressing the brake pedal to maintain speed transmission down shifts to "4" and often to "3". This occurs repeatedly, but is not predictable. When reaching the bottom of the hill and attempting to engage the top gear, I must either (1) depress the gas pedal halfway and release it or (2) manually move the shift lever from "d" to "4" and back. This downshifting is abrupt and unpredictible. I am concerned if it happens on snow or ice, that the vehicle will lose traction and slide.