I am starting to agree with you. I've tried the config described in other posts with one computer configured as a witness and several others configured as miners, all sharing the same private key.  What happens is when one of the miners finds a POW, it gets "missed" and the steem goes poof. When I use the standard config of one account as both the miner and the witness on just one pc it works. So my conclusion is if you want to mine on multiple machines each one should be configured to run under a separate account, and the same account should be used for the witness and the miner.