Panasonic 58dx900 - delayed audio

hi team, bit of head scratcha here

Picked up a new Panasonic 58dx900 over the weekend, i have a issue where when watching sky the audio is ahead of the picture, only on the dolby digital setting, if i change it to output pcm on the sky box audio is fine.

There is no delay option built into the TV (i need a negative delay, not positive) the TV is connected to a soundbar via optical (soundbar only has a positive a/v sync option) , previous TV all synced fine.

Temp fix is to pause TV every time i change the channel then it all syncs up.

Sky box connected by HDMI, then optical into the soundbar, tried with ARC and it did the same.

All picture/ sound processing options are off. all firmware's are up to date, doesn't do it with Netflix/Kodi etc.

Any ideas? done a bit of googling over the weekend and it seems to be a common issue.

Thanks

Blair