I have a 96 chevy k1500 4x4. About 3 months ago my transmission started to kick in and out of overdrive. As I'm driving about 60 my transmission wil shift gears jus fine but once it goes into overdrive it will back an forth from overdrive to 3rd gear. Once in a while it will shift perfect and not act up. Sometimes it won't go to overdrive and it will stay in 3rd. Does anyone have any idea what I need to do to fix this? Is there a shift sensor or gear/rmp sensor that could b bad? I use the heck out of some gas when it does this.