Jimmy Transmission

I seriously need help with this question. I have a 1987 GMC Jimmy 2.8 automatic. I wanted to park my Ford Ranger for the winter and i found this. It is the biggest mistake ever. The transmission is acting really funny. When its cool, it runs like a new one. When it gets hot, after about 30 min of driving, it goes crazy. Going about 45 miles an hour in overdrive you let off the gas and it down shifts to 3rd or drive...but u floor it and it downshifts back to overdrive. It should be the opposite. It gets annoying and acts confused. But it shifts very smooth. No jerks or anything. But if u press the pedal half way down it downshifts and upshifts repeatedly. Does anyone know why this does this or what it is? Im pretty worried about it. Thanks