Alan:
Perhaps your new 5th GPU is bad? Have you seen if it runs isolated on its own? I don't think its smartcoin related, as I've personally seen an 8 GPU system running smartcoin with no problem. Do you have enough system memory? Did you double check the devices stored in the database?
I have 6 cards, and changed the riser cables back and forth and the order of the cards back and forth. All cards ran before perfectly in two separated systems.