Cavalier erratic shifting

My 1999 Cavalier shifts sluggish into reverse when cold fine once it warms up. at a speed of 40 to 55 and Tec at 2500 it shifts up and down causing the car to surge, if I maintain acceleration thru to 3000 rpm and speed 65 car runs fine. trying to maintain 55 is difficult. Back ground engine and transmission both have 15000 miles on them is this a sensor problem or a sticky solenoid problem?