I have to keep rebooting my dbox to see the picture

[sidneysides]

I have a Sagem Dbox2 and am having a problem.
Whilst flicking channels after a while some of the channels don't display - black screen. Sometimes sports and movies, sometimes others. I've tried leaving it for a minute on Sky Sports 1 but surely the keys don't roll every few hours?
As soon as I rebbot, they appear again. Any ideas? It is a Sagem box with Sportster pro image but I've tried lots of images - still the same.

Just tried with a different Sportster image and I am getting the same problem. This is what I've tried so far
Sportster 1.75 - It's Ok if I'm just watching a chnnel for a while, but if I start flicking though, eventually I just get a black screen for most of the channels. If I reboot, the channels display fine
Sportser 1.7 - Exactly The same
Sporster 1.08 - Takes HOURS to get partway through the channel scan
Commando 6 - Will only display the freeview channels. Keys don't seem to update even when I leave it on Sky Sports 1

update
I just tried the linksat beta 4.5 - same problem. Its OK if you are watching a channel, but if you start flicking between them quickly, you lose the picture and have to reboot. It's as if it forgets the key to decrypt them.