Before I bring my van to an untested repair shop I was hoping for some help from the list.  I've had bad luck with repair shops in my area and would really like to avoid taking it to one.  I'm reasonably handy but I don't even know where to start with this problem.

My problem is that the van is backfiring when it is starting. Not all the time but every fifth or sixth time.  It also seems to happen more when the engine is hot.  This seems to be causing my exhaust header on the left rear cylinder to slowly be blown apart.  I've tried repairing the header with a patch kit but every time it backfires the repair is blown apart.

Here are my guesses as to what it could be.
1. Leaky fuel injector on this cylinder. While it is sitting it leaks gas and on start-up it backfires.  This seems to fit the symptoms and is my pick but how do I test this? 
2. Running rich when it is starting.  This doesn't explain why only one cylinder would be having a problem.
3. Leaky exhaust is causing the backfire. (Is this possible??)
4. ????

Any help would be greatly appreciated,

Ron Maxseiner
1985 GL Stinky Pete