when i first started i used a FM modulator and everything sounded good but a month in it slowly went downhill and i was getting so much noise it wasn't funny. It sounded awfull. I got a HU with aux inputs and that solved the problem. If you can pull out your system bring it inside and test the sound on computer speakers. If it works inside its your fm modulator.
As for random reboots.. might wanna check heat in the case.. install at least 1 case fan for output and punch some holes in the case if you made one. Ideal you'd want one intake and another outtake.
Also might wanna run a memory tester on your ram