Well, with the metal rack under the computer and the metal handlebars and maybe/maybe not the metal spooling rig on the back of the ATV (but certainly the metal frame. Why not just stick a good (or beefed up bluetooth unit out the back of the quad and route a USB extension cable up to the front mounted computer?
This way the antennas can all see each other directly without any metal inbetween. I think that is the best chance for success.
The other thing is the ATV ignition system (probably located somewhere between your knees). Even though it doesn’t seem to affect at idle, it might be that bluetooth can find a happy channel without interference when the the ATV is idling, but when you are driving and the throttle is moving up and down, maybe the interference is more broad spectrum and none of the bluetooth channels are free of interference.
I would put someone on the back (sitting backward and holding the computer). If that works without dropouts, then mount the computer back there or leave it up front and run the USB extension to an external bluetooth adapter out back.