Jeep Wrangler Issue

After a clean flush and fill of the radiator and new thermostat this problem still exists on my 2013 Jeep Wrangler:
While going up hill ONLY, the temperature will go from about 50% (normal) to about 60-70% then once the vehicle returns to flat road it will go back to the 50% line on the temperature gauge. The Jeep has about 30,000 miles and it just started to do this within the last 250 miles. Thoughts?

Could be the early stages of head gasket failure.
DIY check: remove radiator cap (when cold!), start engine and watch for bubbles as the engine warms up.
Of course a mechanic can do more sophisticated tests.

Your radiator may still be partially clogged or impede flow somehow. Have you checked out the cooling fan as well?

Clearly the cooling system is not doing its job. A good mechanic can quickly determine where the exact problem lies.

Have you ruled out a small head gasket leak? That would put hot gasses into the cooling system and raise the temperature.