Its the radio that is going into protect/overload mode, requiring a full power cutoff before it allows you to play again. Reasons could be:
It is overloaded. If you have coupled two speakerunits on one output, you halve the speaker impedance from 4ohms to 2ohms, and when you turn the volume (voltage) up then at some point the radio cannot deliver enough current, and it goes into protection mode. So make sure you only have one speakerunit per output. ( NB separate tweeter and woofer counts as one speaker so separates on the same channel is not a problem)
It could also be that one strand of speakerwire is shortcircuiting to the other pole. That would happen if one has done a dodgy exchange of speakers, or if the isolation on the speakercables are 'worn' at any point from the radio to the speakers.
Less likely but also possible: Supply voltage to the radio is too low. Check for it to be minimum 11Volt at all times, and check both the [+BAT] connection and [Acc] connection. (And check that the ground/0V connection is strong/good enough.)
Finally as gtvtommo subtly suggests: the radio is broken, replace it with a new one.
Last edited by Nrik; 03-06-12 at 07:07.
Reason: fixed typos, added comment about ground.