The transmission is just now slipping. But will still run o.k., you just have to let the throttle down a little to let it switch gears.
Also, I had to replace the timing belt, but wasn't a big deal, nor did it involve a lot of money because the engine was so easy to work on.
I love my Mustang. It is my first car, but I have driven many cars. Some worse, some better, but my Mustang is a great in between car. Great on gas, and very reliable. It has never let me down.
Before spending any money on the transmission, change the filter. I had the slipping problem and all it was was a dirty filter. It shifted perfectly after putting a new filter in.