wondering what would have caused this

[linadin]

funny thing is have not pullled anything over 15k since the head was put on last feb. truck has never been over 200 deg water temp. ext temp hit 400 a few times. but i always waited for it to drop below 300 before i shut it down. coolant has not dropped at all. i run dedicated in nc,sc,va,ga very few hills i have put about 2k miles on it a week. i am starting to think they may have done something to the head gasket when they put the new head on. engine does have over 800k mile on it was planing on having a overhaul done next Jan. Just glad it still had a warranty still funny it acted up the day before the warranty ran out normally things happen just after the warranty goes out