How are you connecting your new tv to the set top box?
Hopefully, you should have a scart lead doing the work, has the new tv got scart or other input sockets (besides aeirial RF lead)?
If AV1 on tv accepts RGB input, use oxygen free fully wired scart cables (argos sell them) to connect the STB this way.
Link here:
http://www.argos.co.uk/static/Produc...er/5343717.htm
If this doesn't cure the problem, please post back here, and I'll help further.