[Bob L]

Well, it seems like you've done a good job researching and correcting as far as I can see. At this point, I'm leaning toward something unusual about your rig, so apologies in advance....

The wiring is MAC>LaCie (fw400)>002 (fw400)? No other Firewire devices at all?
The LaCie is formatted per the Avid spec? No RAID?
Do you have anything connected to the Ethernet port?
Which video card is in the machine? How big is the cache?
One monitor or multiple monitors?
I assume that PT is the only running app, aside from Finder?
Wired keyboard and mouse?
You've checked the installed plugins against the Plug Compato list?

And just to confirm, a new session with no tracks at all exhibits the twitchy counter? If you make a click track only, does it play OK or unevenly? Is the playhead traveling smoothly or jumpy?

You get no error messages at all?

Sorry for the barrage, just looking for non~obvious causes.

Bob