Dell 15R 5520 Issue - 7 Beeps - Doesn't start every time

My Dell 15R 5520 Laptop only starts up sporadically. When I can get it to start up, it functions properly. Sometimes I can even restart it successfully.

The rest of the startup attempts (75%) result in the buttons above the keyboard lighting up and then going dark followed by 7 beeps.

After some research, the seven beeps indicates a CPU issue.

So far, I've done the following, without resolution.

-Ran System Restore back to before the latest Windows update.

-Ran the startup diagnostics (no errors found)

-Reseated the processor chip and replaced the thermal compound

-Reseated the memory chips, as well as replaced them

-Tried a different power adapter

I'm stumped since I cannot identify the specific problem, and since the problem isn't consistent.

Any suggestions/advice/help would be greatly appreciated!