I went to
www.standardhorizon.com and checked out my online manual which is going to be similar to other "fixed mount" VHF's in installation.
They rec to temporarily mount it to test for interference with the compass and other electronics if close. Also the antenna may be too close or if angled could give interference as the antenna broadcasts energey horizontally and not upward.
They highly rec that all VHF antennae be mounted at least 2 feet above your head for EMF safety which I have heard can cause eye damage but may effect electronics too.
Might check the solder job on the antenna plug going into the back of the radio? Where's the antenna mounted? Is the compass effected by vhf keying?
Keep us updated, you sound real close to finding the problem.