I've got a 97 Maxima with 120k miles, and I've noticed that the engine seems to be weak. It accelerates poorly, and struggles up a hill much more than before. If I really floor the gas off a stop, it tends accelerate and to shift to 2nd, but then back to 1st as it struggles. However, the idle seems fine and there are no hesitation or pinging problems.
I've recently replaced the air filter and cleaned the throttle body. The spark plugs and fuel filter were replaced 30k ago. What could it be -- dirty injectors, old fuel filter, or something else? Thank you!