Thanks for the further replies. To answer your questions:
I've replaced the camera audio with a external stereo track. I wondered if plugins might be the cause and so disabled all the default audio ones, plus I'm not using any visual effects either.
I'm playing back the files in Windows Media Player and the glitches are always in the same places on playback. Interestingly, on the original clip the glitch was always in the same place on every render.
The hard drive hasn't been defragged but is brand new (about three weeks old) with over 200GB of space on it.
I suspect, as Shoestring says, that this is something to do with how Vegas is implementing the MPEG2 codec. Googling something like "Vegas MPEG render glitch" brings up several unresolved forum posts with exactly this problem including one chap who is rendering fine with an older version of the program but not Platinum 10.
When I tried rendering to AVI and then starting a new project with the resulting file and rendering that to MPEG2 it all seemed glitch-free.
Frustrating, but I'm glad to have found a workaround. I'll try reinstalling at some point to see if that resolves things definitively.
Many thanks for all your input...