[mythtv] odd behaviour - fast forward at speeds greater than 3x

[Simon Kenyon]

i have this problem with playing recordings
if i fast forward at 3x - all is well
if i try to go faster than that then the screen freezes with the occasional 
blocks changing

however, if i rewind and fast forward (at any speed) it will work up and until 
the last point of failure.

that is if i watch a recording or fast forward through it - i can then fast 
forward at any speed. but only if i have played to before.

if i exit from mythfrontend and go back in again i am back to square one.
nothing is being written to the database during the playback. it looks like 
something is being cached.

anyone seen this behaviour and know of a cure.
another data point is that the recording works fine on another frontend and on 
the backend itself.

recordings are from a pvr350 (s-video input)

regards
-- 
simon