Poor reception quality in PVR

My new HUMAX 9300T has poor video/audio quality for all the channels on Multiplex 2 (ITV etc) - evident in both "monitor" mode and playback. By poor quality I mean picture freeze/break-up, sound 'skips' every few seconds- just like a dirty/scratched DVD. Basically these channels are unwatchable via 9300T but absolutely fine when TV connected direct to aerial.

TV is Samsung LE32A556P HD digital. Connection from 9300T to TV is via HDMI (SCART has same result). Local transmitter is Winter Hill (Digital switchover imminent). Communal antenna distribution amp system in apartment block - no reported problems by other residents.

Humax support suggested default reset and manual tuning, which made no difference. Also suggested signal booster, which I have tried, without obvious improvement. Humax hinted that TV may have a "better" tuner.

Signal monitoring by 9300T (system menu) indicates low/variable signal quality (40 to 90%), but my TV has no problem at all. No problem on other multiplexes either 9300T or TV (Signal quality 90%)

I am contemplating returning 9300T and switching to different PVR, but unsure if another brand will be any better.

Does it make sense that PVR tuner is inferior to TV tuner?

Does anyone have similar experience and a solution??