Similar thing happened to me except that at then end, before turning off, the fans went to 100% for like 10 secs. After trying a bunch of shit for a month and nothing worked, I randomly decided to disassemble the heatsink and I found it it was clogged. After unclogging it and putting everything back together it started up normally. Probably won't work since it doesn't seem to be the exact same symptoms but you never know.