Sounds like a bad thermostat. Easy do-it-yourself repair.
Gravity.
Using too low a grade fuel. What is good for normal driving may not be good for hills where strain is put on the engine. think about the spark plugs, wires and dist cap if you have one. often the demand will cause spark to jump elsewhere
Could be a bearing going bad.
what causes engine to missfire when warmed up on my 2002 accent
A universal joint on the driveshaft going bad.
because it is going uphill and the engine has to work harder?
its the trany going out.
That means that the computer see's a problem with the emmission system are the transmission. Need to have engine scaned with a engine scanner to see what is going on.
It is not, but the engine works harder. You generally want to keep the RPM's as low as possible while driving. If you are going to be driving for a long time at the same speed, then it's better to enable overdrive no matter how fast you are going. Not only will your engine last longer, but you get better fuel mileage!
Did you replace the oil pump? If not then that would be suspect. Also install an AC/Delco filter before going any farther.
The CV joint being loose is one of the main causes for a car shaking while it is going uphill. However, a faulty AC and faulty engine could also be the problem.
probably the governor going bad