How did the Roman Emperors manage to successfully place themselves as Pharaohs of Egypt? It seems that such an ancient and religious title would cause a lot of resentment if it was placed upon foreigners who didn't intermarry into the existing dynasty.

It's important to remember that when Egypt became a Roman province after Cleopatra VII's death, it hadn't had an ethnically-Egyptian Pharaoh for more than three hundred years; the Persians conquered Egypt in 343 BCE and took the title, followed by the Macedonians eleven years later, and the Ptolemies - who immediately succeeded the Macedonian Pharaohs - were Greek. When the Roman emperors took the title of Pharaoh, they were merely continuing a centuries-old tradition of foreign leadership.