Black display on iMac but external display works

My iMac's display functions normal through boot up but then goes black anywhere from 10 seconds up to a minute or so after fully loaded. It works perfectly fine on a external display. Often when I leave my computer to sleep and come back to it, it will once again come back on for a short amount of time.

This started gradually probably 6 months ago. It would go black after using it for an hour or so and over time would go black quicker and quicker.

I brought it to Apple and they determined it was a backlit issue, which they replaced. It worked for maybe a week before it started happening again.

Any help would be very, very appreciated! Thank you for your time in advance.