Pi Update 2:
So for those interested.
I am pretty sure it is not power but an issue when the communication. The ML2s trip-up when the pool and Miner get out of sync (crude so more elegant explanation welcome!).
So i am mining a lowdiff coin on a vardiff port. If i put it on med-diff start on the var-diff or low on the vardiff, the ML2s quickly start throwing accepted shares at the pool but just as quickly start to mount up the hardware errors, or start to slow down. This coincides with old share or pool issued old block messages at times, but can also be when the pool starts shifting difficulty a lot.
If i mine a higher difficulty altcoin or using the high vardiff port or a high fixed-diff port, errors remain below 1% for all ML2s at the higher 768 Hz frequency.
As everything else is the same it is pointing strongly towards communication.
When mining a low diff altcoin that is finding blocks really fast on the pool the setup will still get out of sync, but at least i am starting to see a pattern.
Most likely this is my kit, and i am sure more seasoned miners may have figured this out, but if it helps a fellow newbie then hopefully there is some help in here.
bye for now
Quick footnote. Since writing this initially some 5 hours ago i have the system stable on a more difficult setting on same altcoin. definetly some for of synch issue between BFGminer and the pool.