Where do I go from here?
Cannot be of much help.
It sounds to me like a bad control board but your repair company says that it is OK.

Perhaps the diagnostics may reveal something.
See the attachment for the tech sheet.

[COLOR="Blue"]On bake setting the lower oven will preheat normally until temp is reached. [/COLOR]
I am assuming that the bake and broil elements cycle back and forth during preheat.
The tech sheet does give some info on preheat for the upper oven (page 6) but does not show anything specific for the lower oven.
The norm is to switch back and forth.

[COLOR="Blue"]The oven temperature sensor has been replaced but was not faulty per my resistance testing. [/COLOR]
Since the unit comes out of preheat and then keeps track of the oven temperature till it hits a temperature errpor I would also say that the oven sensor is OK.

[COLOR="Blue"]The control board part# 8507P302.60 was sent to CoreCentric Solutions for repair two times and I was told that it tested out completely. They could find nothing wrong.[/COLOR]
I do not know how this company checks out a board so cannot comment on the likelihood of them being wrong.

Double check to be sure that the control board is [properly connected to the elements. I am not sure that this is even possible as the connections may be keyed so that a mistake cannot be made.

Have you checked that the upper oven runs correctly including convection mode?