Yeah. Broadly, for me, it's due to my machine having been on for a few days... after a quick reset it jumps from hanging around a woeful 5fps to between 50 and 110. The previous Singularity demo behaved similarly (or rather I should say that my machine behaved similarly with the previous demo... although stuff like Vice City does not suffer from this crippled behaviour). I've noticed some of my own Blitz-coded stuff breaking down like this too so it's unlikely to be anything Halo's done; just a Blitz foible with certain hardware/driver combos I'd guess. Doesn't a reset help any?
Actually, that worked.. Usually my machine run for a few months before it needs a reboot, however it seems that this requires one. Ran strong 60 fps (refresh rate max) the entire time.