Check the fluid level and make sure it is full and clean. If it looks clean but it is low, be sure to top it off. If the fluid has shiny flakes or is very dark and smells burnt; I hate to say it but it sounds like it is time for a rebuild or replacement. Typically once a Protege trans starts slipping the damage has already been done. I highly recommend bringing this to a professional transmission shop so they can check the pressures and operation of the valve body. From my experience working on these cars and owning several, the transmissions are very finicky.
However, I did purchase one for junk that the owner said the transmission was slipping. After I bought it I test drove it and found that it wasn't slipping... it just wasn't shifting into drive. A little testing and I found out the Air Flow Sensor was reading incorrectly so the car didn't "know" to shift into drive. I swapped the sensor with another car and it ran excellent. So please make sure it really is slipping or if it is just not slipping.
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