98 ford contour transmission???

my car is reving high, it wont change gears (auto transmission) until about 6,000 rpm, so i havr to let off the gas and coast intill it shifts, also the overdrive keeps coming off, i will be driving along ad the overdrive off light will start flashin, when i havent done anything,, i recently changed spark plugs and wires the same day this started if that means anythin i dont know, any advice or thoughts will help, thnx