damn exhaust LEAKS!

check to make sure the flanges are still nice and straight is my standard answer to that but maybe thats not the right one at this point since they sealed up before, i would go for the MrGasket ultra seals, i have used them many times and even reuse them more than i ever thought they could last and never have any leaks!
if you still cant get them to seal you could also check so that its not your exhaust pulling the headers in some direction that makes them bind and not seal up
i never understood those coopergaskets seams way to hard to realy compress in i a good way,but i have never tried them and im not even going to start on what i feel about standard white papergaskets..