And we've finally found the problem. You cannot run 500W through an antenna mounted using a magmount. First of all the coax isn't capable of handling 500W so part of your problem is the insulation in the coax isn't up to the job so you've got arcing internally in the coax. Secondly the magmount doesn't have enough capacitive coupling to give a good ground.
This is what trying to make 500W work with a magmount is:
:bdh:
Now we've got that out of the way, how to sort it? Your only option is to drill a hole in the roof and put in a fixed mount if you ever want a chance of making this work properly. You're also going to want to be doing a load of bonding as well.
My own thought is your radio puts out 80W. That's all you need. Whats the difference between 80W and 500W? Maybe 1 and a half S points on the meter. But as you're finding out the hard way, running a 500W amp requires a hell of a lot more doing than just throwing a magmount on the roof. Absolutely everything has to be top notch with the antenna installation.