i have a 97 4.0 auto 4x4 and then ever its cod out side like under 40 the rpms will bounce from 500 to 1000 and some times die also wen slowing down like under ten mph the rpms will jump again and the whole truck will buck and at idel if it does it will go from l050 rpm to 1100 the whole time and the truck will rock a little. i dont know whats wrong with it if its warm outside itll drive just find but te weird part is if i turn on the heater and the compresser cut in itll act a littl bit better? i got no idea if anyones had this problem let me know so i can get it fixed . thank u

 

throttle body idle air control valve is sticking
remove the throttle body and give it a thorough cleaning inside and the idle air valve bore as well

 

Check what cheese man said.

There also could be a split/cracked hose or rubber elbow (vacuum line). Check all the hoses on your tuck and make sure they are sound.

 

good catch , i totally forgot about the vacuum brake booster elbow vacuum hose

just take the booster hose off and wrap it with electrical tape and re-install