Shifting Problem on '95 Merc. Sable

1995 Mercury Sable 3.0L w/ roughly 40,000 miles (engine a tranny rebuilt a few years ago) and automatic transmission

When driving at random speeds of 35MPH and up the transmission seems to downshift while driving at a steady pace. There is no warning for this downshift and it will sometimes do it multiple times in a row. It will also get stuck in the lower gear for a few seconds at times to where if I touch the gas the RPM's will sky rocket. The car will even do this if I have the cruise control set. It almost feels like it is something electronic telling the car to do this as there is no akward noise coming from the car when it happens.

I have changed the EGR pressure sensor, speed sensor and cleaned all grounds (to possibly fix another problem). I have also had the car on a diagnostic test and also the ignition module tested with both coming up clean.

Any input would be greatly appreciated as I don't have much money and this will have to be a job I do myself.