Sounds to me like you're overdriving the deck's internal amp.
Maybe heat too. Might want to check your wiring and other stuff around the deck and make sure the radio is able to dissipate as much heat as possible.
Or, you could get yourself a nice amp (my personal choice). In reality, any head unit will only put out 15 watts rms per channel.
With an external amp, you'll notice a big difference at all volume levels too. Better dynamic range, more powerful bass, more volume.
