PSS
It is impossible to say what has gone wrong but from a Boeing course I attended in 1998 buffet onset is, say, lift coefft = 0.69
If you were heavier than about 241600kg then you maybe had no business to be at FL430 M0.84 and the sim quite rightly stung you though its warning signs seem a bit odd. However FL390 should be fine. Try that.
I hope that makes some sense. Have a look at the documentation they supplied and keep within the flight envelope. If you poke the bear eventually the bear pokes back.
Keep up your interest in simming but do remember your pastime is only based on as much data as they can sell for $50 and make a profit. So most of it is eye candy, certainly not approved by the maker and can therefore teach you bad habits.
All I can say is read your manual and stick to it and in this case a better forum might be for simmers. But nice to meet you and good luck.