My MkV also went crazy with crackling & popping relays. Pretty much out of the blue.
I'd been gigging with it since I got it in Summer 09 with few or no issues.
After the relay problems started, I tried swapping the cable from my Roadster.
The cables are very finicky. The Roadster had similar issues. At first the other cable
seemed to calm it down, then it went crazy again and it was unuseable.
Finally took it in for servicing. My tech applied a Factory engineering update to
the Footswitch circuit. Even though it spec'd out ok, he wasn't happy with the workaround.
He gave me a new cable as well.
After I picked it up, I fired it up and, Bingo! Exact same problem!
Tried 2 different cables with varying results. The key is to make sure the cable
is seated very securely at the amp and FS. Do this before turning on the amp
or else it's relay city.
I don't like the feel of the cable connection at either end. I just think the barrel isn't long
enough to make proper contact. I thought it might be the plastic jacket that was too large
to fit in the opening, but on closer inspection, it had plenty of clearance on the FS side.
Once I position the cable and FS on the floor, I can't even breathe near the cable or it
goes crazy. As long as the cable is secure and doesn't get moved at all, it seems to
work ok. But I always bring a spare amp just in case....
Good luck!