Flange bolts.. That's a tough one. No one smart torques them. They do like you've experienced. Why? Heat. Heat will expand things, and then later it'll cool down. That constant expansion/contraction is what causes loose bolts and an exhaust leak.
That said, always apply anti-sieze. always.
Assuming you're using a fresh gasket.. I tighten till the gasket has compressed almost 50% it's thickness.
I try to almost always use a split locking washer. They are single use items because of the above heat issue.
Torque values, I'd wager are in the 30ft/lb range, upwards of 40ft/lbs. Or till my wrist clicks, which is anything over 25.
