does this problem happen consistently? does this problem happen anytime while driving? is it one speaker that doesn't work?

you can play with the fader in the car to isolate which speaker is working and which isn't. start by directing all the sound to the front, then to the left. from there, check all 4 speakers to see which one isn't sounding right.

if it's all speakers that cut out, i'd check the RMS each speaker is set at. you could be overloading your headunit causing it to turn off at peak power demands.