If the fault disappeared when you hit the TV, it's likely just dry soldered joints or faulty electrolytic capacitors in the field output circuit. It probably wouldn't cost too much to get it repaired, as field faults are usually pretty straightforward to correct.
It can only get worse if left, which could result in a more expensive component failing, so it's best to bite the bullet now and call out an engineer asap to rectify it.
Cobweb, December 2004
