[mythtv] What am I doing wrong?

[Gavin Band]

Dear all,
I have a Gentoo linux box with MythTV installed.  mythfrontend --version
reports:

> mythfrontend --version
Library API version: 0.19.20060121-2
Source code version: exported
Options compiled in:
 linux release using_v4l using_alsa using_jack using_ivtv using_dvb
using_dvb_eit using_x11 using_xv using_dvdnav using_xrandr
using_opengl_vsync using_frontend using_backend

The backend seems to work correctly, but I can't watch anything (live or
recorded) in the frontend because the display repeatedly pauses - then
catches up - then pauses - then catches up, etc.

The log prints out lots of "prebuffering pause"s.

This is with a fastish laptop, Radeon Mobility 9600 with open source
driver, Freecom DVB-T USB stick.

Have I compiled / set something wrong?  Any help appreciated.
gav.