VOR/LOC Intermittent with Garmin 430 Source
 

I own an aircraft that has experienced a VOR/LOC failure consistently in flight but when on the ground, it works fine. It is being sourced by a Garmin 430. It fails in GPS mode, VLOC mode. Doesnt matter. Any mode, any station tuned, any flight plan, no flight plan. Always broken. Get it on the ground, the avionics guy tests it, it passes every test. Been to two avionics guys now. Suspect a bad NAV board. Probably going to replace it but figured i'd just see how many other people have seen a problem like this.
A broken VOR - YouTube

Can you get it to act up on the ground when doing a run-up or during taxi?

You may have a chafe on one of the resolver, deviation or flag lines between the 430 and the CDI which shorts to ground with aircraft vibration. In a GNS430 installation, all of the signal lines are referenced at +2.5V, so you'll get full-scale deflection if one side goes to ground. This could also be a bad solder sleeve on one of the screened cables.

Love these problems...nothing wrong with it...it just doesn't work!!
Have you checked the antenna cable and connection?'
Possibility airfame stress inflight.
Happened years ago on a Swissair DC 9. BNC connector at the antenna was not locked/ disconnected during landings.
Don't overlook the obvious.
Good Luck
Putt