1977 mustang II with c3 automatic shifts hard and at random. Why?

I recently acquired a 1977 mustang 2 with a c-3 auto tranny and it started shifting at random when cruising on fairly flat even blacktop at around 40 mph. This started the second time I drove it. The third time I dorve it (second time on the interstate) it started doing on my way home (32 miles each way) and it gets worse every time I drive it. The shifting is rather harsh and causes the motor to go from normal speed to redline almost instantly but seems to happen more often when the operating temperature is normal vs cold. I think it may be a vacuum modulator issue or a hose but I'm not sure exactly where those hoses are and as such am having trouble getting a visual inspection of them (and the modulator). The only reason I suspect these parts at all is because I am unfamiliar with these autos and have read about them. Also, all the rubbers under the hood were new (i.e: belts and hoses) and all the fluids were fresh (save oil). The motor was the original 4 cylinder but was converted from a two barrel carb to a fuel injection system. If anyone has a vacuum diagram, experience, knowlege, or literature on the subject and would be willing to share it I would be most apprecciative.