Well, this is a long shot, but if your voltage regulator isn't regulating voltage, the voltage may get too high for your radio at a certain RPM and it shuts itself down.
Well, First of all sorry for posting in the wrong section(maybe i need glasses) and secondly i like your sense of humour round here !
Double thankyou for the helpful replies , 6 pack i hope you are right as i have ordered a new regulator on the strength of it !
I did have a similar problem with my 66 stang, but that was a mechanical regulator with points in it,fitted an electronic one.....fine !
My 72 cuda already has an electronic one..........but it could be faulty i suppose.