Gets hot going up hill cools down on level on hiway

replace your coolant and add plenty of anti-freeze, make sure your thermostat is working ok. when bleeding the cooling system make sure the filling point is at the highest level, run for 10-15 minutes with the cap off to ensure all the air is out.

First You have to bleed out all the air in the cooling system.
If that doesn't fix it then the system is probably clogged. Also the thermostat could be not opening.
Hope this helps.

check your codes first.

if there isnt any leak in the cooling system the fan clutch is changed and the fan along wit the thermostat dat mean u can probly have a clogged radiator with poor cooling system circulation or u might have to take it to a repair shop to check if the head gasket is blown

The jiggle valve is just to assist in ensuring no air trapped behind the thermostat.It is important but only really at the bleeding air out stage.When it gets hot,throw the heater on,it should get hot and the temperature will come down or stabilise.I would recommend removing the radiator and having the top tank removed and the core tubes cleaned thoroughly. Refit it and ensure that you fill it slowly to allow air to escape.Use a good quality coolant too.Get back with your findings if fix still needed and I will endeavor to pinpoint repair needed.

Check your transmission fluid level. The fluid could be low. This will cause the car to slow down especially going up hills.

fuel pump may be going bad have pressure tested on vehicle when warm also egr valve may be going bad or has bad carbon build up lack of power a miss will give u a trouble code[check engine] as well as egr valve and other emission parts but fuel pump usually does not

From the last portion about your drving conditions, it is normal operations....it gets hot, especially going up hills with the A/C on. Sometime the water pumps start to weaken but also a clogged radiator, slowing the flow of cooled coolant through your motor or even through the radiator....check to be sure you have the proper level of coolant also.

try replaceing your temp sencer. some time they are near the goose neck or on the block ok . and see if your fan sencer is working. good luck

Does temp gauge go up when this happens ? When was fuel filter changed last? If not recently, I'd start there.