Going back and forth from "D" to "O/D"

Car shifts from first all way through but after it has to hear back down it constantly jumps from drive to overdrive and back and forth, sometimes, will not grab a gear at all then you will have to just cruise at a slow speed and barely push gas to get it to go. Overdrive light does flash on the dash at times when it does this and if you push the overdrive off button on shifter it seems to help some but it will still try to go into overdrive even though you have selected the off button. Please help, we are hoping this is a simple fix.