Theres a # of things that could cause that problem :
Ignition timing high,---Check and adjust.
Engine overheating or running hot. Check.
Knock Senser bad, Check and replace if needed.
Electronic Spark Control Module shorted are open circuit. Check and replace if needed.
BAD FUEL ,
you stand and pedal like what you would do when going uphill
I would suppose it would matter if you were going uphill or downhill.
It could keep the engine from running, but not the valves from moving.
To adjust the valves on a Chevy Celebrity 2.5L 4 cylinder engine I would recommend that you consult a manual, either factory or Chilton or Haynes. There you will find not only how to do the adjustment but the settings to use to adjust the valves in the correct closed position.
Is the vehicles transmission downshifting to a lower gear as you go uphill? If not, that would be your first place to look. After that I would suspect that your engine timing is not autmatically advancing as it should be.
Change the fuel filter first then have the cat convertor checked out .
A drop in speed while going uphill is normal for most vehicles. If the speed is more than 20 mph slower than when first ascending the hill, there could still be a problem with the transmission or an oxygen sensor.
A miss fire is often caused by either a bad or fouled spark plug or spark plug wire. It could also be caused by a failing coil, it that case two cylinders would miss fire. I would look at spark plugs and wires first.
You either have the WRONG push rods in it, or you are not adjusting the valves correctly. There is a special way to adjust the valves and it would take to long to explain it to you. They don't just tighten all the way down. You need to get a book and READ about adjusting the valves.
If the slope is 'uphill' then the car is going faster
It could be the timing chain/sprockets marks are not lined up correctly or the valves are not adjusted correctly.
I would be suprised if you have adjustable valves.