I just got a 1999 s10 4.3 automatic that only has 51k miles on it. When I got it the intake manifold gaskets were leaking and I'm just now getting it back on the road. The truck seems to run perfect, idles smooth and has plenty of power if you get on it, but when you are cruising in OD at 55-65 mph and come to a slight hill it just feels like it has no power and seems hesitant to downshift unless you floor it. No codes set. I honestly can't tell if its some sort of misfire, or maybe a torque converter shuddering I just know there is a lack of power and some very light vibration. Truck doesn't have a tac so its sort of hard to tell what its doing. Plugs/wires appear to be original and I assume the atf is as well, although it looks very good on the dipstick. Any thoughts?