Lack of Response and Bogging Down

Hello. I'm looking for some problem diagnosis for a 2003 Mazda Tribute LX, 6 Cyl., 4WD with about 154,000 miles. The problem has been while driving I have a hard time keeping a constant speed especially around 35, 45, and 55 mph. When I try to keep a constant at those speeds the car will bog down or with a very slight lift off the pedal the rpms will rocket to 3500-4000 and it will shift into another gear and then bog down again and rpms drop to 500. Another symptom is I will try my best to keep at speed and it will slowly lose speed or if I take my foot of the gas, keep engaged in that gear and continue at that speed for a bit.

In July 2015 the transmission was replaced with a used transmission. I have since that time also replaced: TPS sensor, air idle control valve, MAF sensor, and all six ignition coils/plugs. I am at wits end here. Thanks.