Me and my friend get huge amounts of stuttering just before the final boss. There is no lag grace message and both our FPS is fine (according to Fraps), the game just stutters more the closer we get to the boss. It starts happening at the exact same place, we've tried it three times, Steam restarts, router restarts, PC restarts, nothing changes. It's rather irritating.
We have only played the Eldar campaign together but we have both completed other campaigns on our own with no problems.
Final mission spoilers
The specific place it starts to stutter is after the second Skull Tower, it progressively gets worse until it gets nigh unplayable after the fourth Skull Tower.
Operating System: XP Professional SP3
Processor: AMD Phenom 9550 Quad-Core Processor
Graphics Card: Nvidia GeForce GTX 280
Memory: 3326MB RAM
Operating System: Vista Ultimate SP2
Processor: AMD Phenom 9950 Quad-Core Processor
Graphics Card: Nvidia GeForce GTX 260
Memory: 4GB RAM
I'm certainly sure our specs aren't to blame.