1998 Ford F-150 "Bucking" or cutting out

My truck will intermittently "buck" or cut out at speeds above 45 mph. Before I had it checked out, it would do this quite frequently. The mechanic changed the EGR valve and another piece related to the EGR.

My truck still bucks or cuts out, but only intermittently, and seems to get worse the more I drive it. When the mechanic initially checked the transmission fluid, it was coal-black. He said changing it would only make the problem worse. He sent me to a transmission shop where, after supposedly diagnosing it, they told me my transmission was about to go and needed a new transmission.

I just want to know what is causing this "jerk", and if changing the transmission fluid will make it better or worse. And, if it's NOT the transmission or fluid, then what else could be causing the problem?