GM-GMC: chevy truck, torque convertor, shifting gears

Question
Hi i have a 92 chevy pickup 1500 4x4 with the 305 motor and auto tranny. The problem is when im  driving the engine revs up and down almost a full rpm from about 2000rpm to 3000rpm when i get in the 35 to 45 mph range but only if my foot is steady on the gas pedal in that range. if i speed up or apply the brake this problem goes away. thanks

Answer
This sounds like a transmission problem to me, which is not something I specialize in.  It is possible that your torque convertor clutch is coming on and off, kind of like shifting gears up and down, it can't find a 'happy' balance.  I would recommend consulting a tranny specialist.
Hope this helps