Weak or no signal on UE37C6000, improves slightly when hit!

My Samsung UE37C6000 has a weak or no signal on 30% of channels. A TV engineer checked the signal quality and said it was fine. I have swapped it with a different TV (a Samsung UE26EH4510) and that works fine using the same ariel socket and lead.

Therefore the signal seems to be ok and the problem appears to be with the TV itself. Another reason I think the TV is at fault (and not the signal) is because if I hit the back of the TV the signal sometimes improves !

So I was wondering if it is a poor connection like a dry solder joint and how to go about fixing it (I have a soldering iron and some tools)