All stock no mods..CS, welcome to the club!
I cannot tell you what amp you have, T-B may be able to help with that, otherwise if you can get to it, you should find that info on the amp itself.
As for the amp cutting out and/or sound quality, i would think either a drop in supply voltage/amperage, a ground connection, or it just can't keep up with the demand and goes into overload protection.
Have you added additional speakers or subs?