RACtask isn't disabled on this system, so whilst that was definitely causing the freezes/continuity errors every hour, perhaps that was more a symptom of something more fundamental being wrong.
Once I've got the other system in the case, I'll do the tests Kessu suggested (Safe Mode, TV card removed) and check the latency again.
EDIT: Hmm, I left LatencyMon running (although the timer has reset and currently shows 02:43) and just had a look and it shows a warning and Highest reported DPC routine execution time as 23379296410126.7500 (netbt.sys). The drivers tabs only shows netbt.sys as 6196 DPC count, 0.288ms though so perhaps LatencyMon is a bit buggy. Actually, I've just checked and I'm using v3 and there's a v4.02 so I'd better upgrade!
Edited by doveman, 28 July 2012 - 10:10 AM.