Hesitating and not getting any faster going up hills.

No overheating issues.I don't see any coolant missing. Replaced the sparkplugs and wires. Placed a new rotor cap, not the rotor. Or the fuel filter. Added fuel injector/carb cleaner fluid with last fill up yesterday. Going to get oil change and other fluids today. Thinking about taking it to a muffler shop and see what they can do. Someone told me there might still be an exhaust leak somewhere.

Still the same, had the vehicle almost 2 mos, and it seems to strain more so than before. Could have been from the exhaust leak before. No power when going up hills, and I am stepping on the gas pedal all the way down. Doesn't die or stall. Stays on, but I don't know what else to do. Has after market parts. Headers, one of those pipe attached to a large air filter. I don't know what else the seller added to it. I know he says it has a fairly new engine with approximately 56Kplus on it. What would cause it to not have power when gas pedal is all the way down going up hill. Sounds like no power at all. Thanks KHLow for a lot of your replies.