I kind of think this is a clue:
“its transponder and other tracking systems were off.”
Also flying a few thousand feet below its last radar fix.
Sounds to me like plane made left turn, decending to get into the east west flight lanes and heading for, well, what could be west of where they were? TADA: Eastern coast of Africa. 777btw has a range of over 8000 miles with full fuel load. I don’t think we have a mechanical problem here. Pilot error or errored pilot perhaps.
Military tracked missing plane for at least 100 miles after it vanished from civilian radar - and are sure it made it to Malacca Strait, Malaysian source reveals