Ok you diagnose this one

[84LX89GT]

Part of the reason header gaskets blow out is because they are thin paper and even a small amount of clearance will blow through the side of them....what causes this is that headers warp up over time. The ones on my car have warped and luckily haven't leaked yet. All headers warp, some just warp less than others. When you take the header off, take a straight edge and take a look across the ports of the header, sometimes using a feeler gauge will help find trouble spots. Also, using a copper header gasket helps alot because it is thick and tolerates more variance in the surface if you decide you don't want to mill the headers down flat again.
One of the reason headers warp is that they heat up really fast and cool down really fast, this tends to warp metal. Also, if you don't torque down the header bolts evenly this will aid in warping the flange.