Likely because ur battery is going low and can't support the voltage your amp needs. There are alot of other things that can be doing this. What size amp is it? And what size wire did you run for it? Where abouts in Ont are you from?
Just make sure you scrape all of the paint off the chassis. I used to live in Ont, my brothers and I used to go to the race track in Peterborough a bunch. Ever go?