tyron.c

...

ru'

Could well be a 'speaker wire shorting to ground. Do you loose the sound from all four loudspeakers when it happens?

What was the state of the wiring behind the HU when you changed it?

Have the 'speakers been changed from standard?

To isolate which 'speaker it is, disconnect one (front right, for example) and drive around to see if it happens.

If it doesn't, reconnect that one and disconnect another one and repeat until you find which one it is.

tyron.c

...

ru'

Could well be the 'speaker terminals shorting out on the body-work?

(er. re-read your post and this seems unlikely if the original units were put back. However, if he changed them then he probably removed the original connector, and so had to sort something out when he put them back in)

tyron.c

...

ru'

Surely it's an excuse to fit some better 'speakers soon though?

Hope it's sorted.

tyron.c

...