94 Firebird has problem but no codes

My wifes 3.4 has a definite problem but the local auto parts place scanner shows no problem codes. If you accelerate slowly you can get up to speed and it will hold 70 all day, as long as you don't come to any steep hills. As soon as you try to accelerate rapidly it will bog down regardless of what speed you're going at the time. It seems like the problem is a function of a certain manifold vacuum or position of the throttle valve. The fact that it will do 70 for extended periods makes me discount coils or spark related parts. ( wouldn't they give a definite code anyway?) Can a TPS , which I believe is just a variable resistor, have a bad or noisy spot in the windings the same as a stereo volume control and send garbage to the brain box? Anybody else in forumland have a similar problem? thanks

have you had the back pressure of the exhaust checked. a restricted catalytic converter could be the cause. other than that i would chack the fuel pressure