I have a Rheem gas furnace. Ignites, Gas shuts off when Heating element dies down..?

Thought it was the flame sensor. Replaced that. Thought it was the control box, replaced that. Still it lights, then shuts off when the element dies down. Don't really have the money for the repair man. Last one cost me a fortune. I have the almost exact furnace in an apartment in my house so I was using those parts to test. Its all hooked up correctly as far as I can tell. Hopefully someone can help...