midi clock latency bug - HELP!!!!!!

I have found the most annoying of annoying bugs. I really hope someone can help. I'm going nuts here.

I have a range of external rack synths, linked via usb to the old faithful amt8. My setup hasn't changed for years and everything was hunky dory in logic 9, however, when I send midi clock to "all" so my external synths can get clock info to sync Arps, etc, theres so much latency. If i disable send to all, all midi devices work and playback with no issue. sorry if this doesn't make sense, I've been at this for hours now and feeling at a loss. Brain is fried.

Ben