Transmission? problems

When the outside temp reaches about 80 degrees, my '00 Focus SE starts to hesitate, and slip in higher gears (not wanting to catch when shifting), and really wants to kill when going about 5 MPH. I was told that the tranny fluid is foaming, and the line needs a flush. I did add some of that TransMedic/sealant stuff, but did not help. This only happens on hot days, and I'm very scared of having it kill on me when I'm doing 60MPH on the highway. Could this be the timing sensor too? Or could it be a leak in the fluid, or does this "foaming" cause these symptoms?????? Any and all help is greatly appreciated.