1994 Mustang slips into neutral while in drive

I've posted this before and haven't had much luck getting this fixed so I thought I would ask again.
I own a 1994 Mustang and after about eight or ten miles the transmission will slip into neutral. I can either let my car coast down to about twenty where it will slip back into drive or I can shift TO neutral and BACK into drive. The transmission fluid is fine.

Are there any sensors that could cause this?