Just a late follow-up after trying the trailer in exactly the same location where it had problems last year:
The new circuit board and new ignitor fixed the problem. It worked perfectly at 10k' in spite of Dometic's comments about it not guaranteeed to work at anywhere near that altitude. Dometic was right about 1 thing...it wasn't a jet related problem.
Maybe this winter I'll put the old ignitor back in and see if the cold causes it to be faulty. If it works fine, I'll strongly suspect the old circuit board.
Thanks again for everyone's help.
dmac