My cars are Escorts, which had a predecessor to the Focus automatic transmission, and the probe could be very similar. Your symptoms could be something as serious as some broken parts floating around inside the transmission, or some metal shavings caught somewhere in the valve body layers, or even a bad solenoid controlling the fluid line pressure in the transmission.
But I would also be checking that its not a bad "range sensor". This is an electrical goody that lets the transmission control module (part of the PCM I believe) know what gear you want it to be in. The only gears you get via the hydraulics are first and reverse. ALL of the other gears involve solenoids being activated to do the shifts. If the range sensor is getting dirty or intermittent, the trans could be leaving one gear, but not going into another - due to the computer not knowing what it should be doing.
Commonly transmission shops will simply say the trasmission needs to be replaced. This is because its easy to say, but its hard to really do any repair to a transmission that is in a car. But try to have the range sensor checked, and while at it, make sure the lines running between the transmission and the trans fluid cooler in the radiator are not kinked shut. This could also cause your symptoms.