I've had this problem going on two years now. I've also posted on it before with no success. I seriously doubt it's the ground. You can try it and let me know if it works. I've regrounded mine several times and even tried some expensive monster cable. I've installed many amps in my days. You'll only notice this problem when you hit a bump or on any hot day or after the radio has been running for a long time - correct? It usually only sparks up in the summer. A quick fix is to pull over and give the center of the amp a lite bump with your fist (with the car on). This will only last until the next bump in the road.
I have a little electrical background and a colleague of mine gave me a good opinion. He believes it's a loose circuit actually inside of the amp that needs to be re-soddered? It sounds legit to me but I haven't found the time to pull it down and check yet. But I'm pretty sure that it's the culprit. You'll be regrounding that amp forever. Believe me. I've been waiting patiently for someone else to run into this problem. You won't notice it on cold days or in the winter.
