I have a 1998 ford windstar and mycheck engine light came on and the code was for the idle air control valve I replaced the valve and the reset the light but the light came right back on after driving a few miles then my transmiission would shift down ih to the first gear while I'm driving sometimes I can shift it to neutral and back to drive and it start shifting normally again something's it takes a while before it starts doing it and sometimes it won't do it at all what could be causing this