Since you are pulling that much current, you should really think about adding a secondary battery in the back of your car.
I say this because with the amps you have, you are really making your alternator work overtime since the one you have right now is most likely stock and cannot put out enough amperage to keep up with the pull of the amps. If you were to get a secondary battery and a high output alt, you would be set. You should get a bigger alt then just 170 if you can.
My best advice on an alternator would be to look into DC Power...
http://www.dcpowerinc.com/
The clipping you are seeing is most likely caused from voltage drop.