Catastrophic downpipe failure, again.

[doubt at inwave.com (Nate Mellom)]

As some may remember, back in May I was having problems with my dual 
downpipe on my '81.  It sprung a leak, then more leaks, and finally both 
pipes busted off the manifold flange, clean off.  I got a replacement 
from a local VW shop, and it looked like it was in good shape.  Not new, 
but usable.  Over the last 2 weeks, I've developed another exhaust leak, 
and on Friday the same thing happened, the pipes busted off the flange. 
What gives?  Is this a common failure?  I'm running a pretty stock 
system, 4-2-1 mani, 2-1 DP, stock flex pipe, stock cat, TT cat-back 
that's been on for almost 2 years.   Now all of a sudden 2 busted down 
pipes in 4 months?  Bad luck, or is there something I should look for?
Thanks,
Nate