I have a kitchenaid KFEG500EWH0 that has recently starting having some heating issues.

When I set it to pre-heat to 400 it will very slowly get to about 350ish, maybe 360 and just hang out there. If I set it to 500 it'll get to 425ish slowly. The other day I had it at 425 and opened it to put the food in, then it immediately dropped back to 320 and started pre-heating again.

Is this a sensor issue? A thermostat issue? In either case I'd be able to replace it myself, I'm just not sure which I should try replacing first.

Any advice would be much appreciated!

Note that both the bottom, top, and rear convection elements have all been tested and are turning on during pre-heat so I am pretty sure it's not a bad element.