It sounds like you're running a 3-module Avalon unit. I run one myself, and it will occasionally drop a significant amount of hash rate and require a restart in order to resume normal speed. It has nothing to do with the pool, it's the hardware. It will generally run 1-3 weeks before the problem shows up again.
After restart problem stay.
They working about 4 weeks, and I first have this problem. In meanwhile, I do not know if it restarts when I was not there.
Also they working on other pools, before, and don't have a problem. Now also.
Have you let it run for _ONE HOUR_ straight? If you have hardware issues and restart, it will take a full hour for your speed estimate on BTC Guild to update. There is *NOTHING* wrong with the pool, this is entirely you, so there is definitely something either wrong in you're setup, or you're not waiting long enough for the estimate to adjust.