My Sony head unit cuts out

Hi, My Sony CDX-5000CD head unit cuts out when I turn the volume
up high (it gets about half way up the scale when it does it). As far
as I can see everything is connected securely. The power output of
the unit is 4x50 & I have pioneer speakers in the front & sony
speakers in the back that should more than happily take this power
output. My earth runs through the radio aerial rather than the
specific wire from the connector. Anything odd about that??? Anyway
im stumped. Any help would be greatly appreciated.

Many thanks,

Mark Rogers