automatic tranny shifting problems

Help I have a 98 ZR2 4x4 4.3 truck that has started shifting back and forth between 3rd and 4th gear at 45 to 55 mph. The truck shifts good when accelerating or driving on the xpressway at 70mph. Than there are times when it runs great at 45 or 55 mph. Anyone have any ideas on what may be the problem?