1996 Saturn SL1 car looses power climbing grades

The car looses power accelerating but it doesn't do it all the time it may do it one day or on the way to work but not on the way back and it does it in any gear except first. only when the motor is lugging like when it shifts up and the rpms are low. You can give it more gas and it doesn't respond until it down shifts. Which to me it doesn't sound like anything mechanical. I have changed the fuel filter twice and spark plugs and wires are fairly new and don't look to be fouled out.which leads to a sensor like a map or o2. and there aren't any codes coming up the light doesn't come on. Which ive heard of o2 sensors fouling out and never send a code. The car doesn't do it on the highway at all around 70mpg just on back roads or mileage under 60mph. Thank you