98 ford contour transmission?

my car is not shifting untill 6,000 rpm (auto transmission) and most of the time i have to let off the gas for a second or 2 to get it to shift, after a few minutes of driving sometimes longer the overdrive turns off, i dont push the on or anything it just does it by iself, when overdrive is off it shifts at a much better rpm, still a little high but better, what could be the problem, do i need a whole new transmission , any thoughts