jittery audio mythtv 0.26 and wheezy

I'm having trouble with very jittery audio on a new myth install under
debian wheezy.

mplayer works fine. (as does everything else I've tried)

Everything I have tried to play (movies, live TV (both HD and SD)) s s
o u n n d d s s
l l i i k k e e c c r r a a p p - TV runs slow too - the only way I
can fix it is select
the NULL sound device - then play speed is normal.

I've tried messing with the buffers in /proc/asound :-

..currently all set to 256 ..

/proc/asound/card1/pcm8p/sub0/prealloc 256
/proc/asound/card1/pcm7p/sub0/prealloc 256
/proc/asound/card1/pcm3p/sub0/prealloc 256
/proc/asound/card0/pcm1p/sub0/prealloc 256
/proc/asound/card0/pcm0c/sub0/prealloc 256
/proc/asound/card0/pcm0p/sub0/prealloc 256

..but havent had any success with anything from 64 to 2014.

I've also made sure extra audio buffering is on in mythtv - but nothing
has made a difference.

My sound hardware is :-

          *-multimedia
              description: Audio device
              product: NM10/ICH7 Family High Definition Audio Controller
              vendor: Intel Corporation
              physical id: 1b
              bus info: pci@0000:00:1b.0
              version: 02
              width: 64 bits
              clock: 33MHz
              capabilities: pm msi pciexpress bus_master cap_list
              configuration: driver=snd_hda_intel latency=0
              resources: irq:46 memory:db400000-db403fff

..its a zotac zbox - one of those atom thinggies with the integrated
Nvidia chip... I just want to use the analog jack at the front to drive
some stereo speakers.

Any suggestions very appreciated.