Becker Grand Prix - random no sound

My SL500 came with a Becker Grand Prix hi-fi with BlueTooth and iPod connectivity

The model appears to be a 544 - (see photo) - although I can not see any reference to this model on the Becker site - so I am presuming its something like a 7990...?

The unit powers up every time and the display works fine - however I only get sound out of it about 1 in 10 power-ups..?

Most times I hear a crackle on the speakers - then nothing

All other functions work - and the no sound issue is the same in radio or cd mode

When I do do get sound - its perfectly normal and it may work for the next two or three starts

Turning the unit on-and-off, using its power button, makes no difference - only full power down using ignition key will get it to power up with sound (when it decides to).

Generally, once I have sound, it will stay on for the whole of the journey - although on just two occasions it has gone off

When I power the unit up and hear the crackle on the speakers - before silence - I was wondering if their is an over-current drawn and the amplifier shuts down..?

I have an electric aerial - which extends every time the unit powers up - I was wondering if that might cause a current surge which shuts the amp down.

But why does the rest of the unit - including the CD player continue to function - but no sound?

I have thought about broken / intermittent speaker cable - but the problem is with all speakers not just one. I have had the unit out and checked all the connectivity at the back - no obvious problems

Any suggestions please..?

Thanks in advance

Mike