What's wrong with my car?

1997 Ford Contour, rebuilt tranny in May. Feb replaced oxygen sensors, July replaced a/c coil and valve gasket cover. Tuesday night stopped at stoplight, light turns green, hit gas, nothing happens. Floor the gas pedal, car goes 1mpg through intersection I pull over green o/d light on and flashing, car dies. Car starts fine no problem but put it in gear and forget it, it dies. Car does need timing belt tensioner replaced, but mechanic thinks this did not cause the problem Tuesday night. Could it be the tranny? Trying to decide where to have it towed to. If tranny its under warranty, if not tranny, it probably goes off the cliff. :(