Problem speed - dvb-s/c

Hi,

I use mplayer with dvb for years now (and thanks! :p) but i always have the
same problem: 

Video are slow sometimes (eg: In serie-tv Lost, where you see jungle with
many tree), there are big slowdown and finally, i got a desynch A/V

Solution ? down the LAVC option...
But the quality are bad...

I would like to stay with LAVC > 7000 but in most situations there are not
stable...

Tested with DVB-S, DVB-C now, many drivers (now kernel driver), many
firmwares, many mplayer version... and always same result...

The CPU just grow to 30% maximum, so its not the problem i think

There are something i miss ? 

Thanks!

Mplayer 1.0RC1
DVB-C
Kernel 2.6.18
Mplayer.sh 0.8.6
Athlon 2800+
1gig ram
FC6