Annoying intermittent sound problem - Rockola

I hope someone can help me with an extremely annoying problem I am
having with a Rockola 443. The problem is the sound keeps cutting out
but there seems to be no pattern at all to this. For example it might
work okay for a while when it is turned on then the sound will stop,
other times it will select okay when turned on but no sound then at
some point the sound will start.

I think the amp is okay, checked across the filter caps for voltage
and at any time I will get hum through if I touch the cartridge
inputs. The cartridge is okay also as is the mute relay.

The strange thing is that if a selection is playing with no sound I
can make the sound return by:

Twisting a lamp so that there is a crackle through the amp
Sometimes making another selection (after the number button is pressed
the sound starts)
Playing with the connectors (although a different connector each time
- not one specific connector)

When I do one of these things the sound might come back on for quite
some time then drop out or it might only come on for a short time.
Sometimes it will come on crackly and low volume other times it will
be clear and normal volume.

Could this be some sort of grounding problem or something? I hope I
have explained this okay, even though I am a novice I just can't
narrow down what it might be or where to start.

Really hoping someone might be able to point me in the right
direction.

Paul Smith
Australia