Transmission slams up and down in gear at highway speeds and sometimes in lower speeds

I have a 98 ford taurus se with 3.0 dohc enjine. When driving at highway speeds it will sometimes (not always) start to slam up and down in gear and the tachometer reads that it is reving 4500-5000 rpm's while it does this. It also slams in and out of gear at lower speeds. I have looked for and fixed a few vaccuum leaks, I changed the throttle position sensor. I have changed the neutral park sensor (the mechanic called it a pringle) I am out of ideas and hope you can point me in the right direction