Assuming that the avionics are working properly, which seems a fair assumption, it sounds like an electromagnetic interference (EMI) issue. A good avionics shop will have a handheld unit which can artificially generate the signals to test your indications (I have witnessed this testing being done before). Start with that - with a solid test signal, you should get a solid indication.
Next, consider that wiring and antenna wiring may be in conflict - has ANY wiring been changed in the fuselage since the 'plane was new? Look there first. It has happened that the addition of entirely unrelated wiring has interfered with presently installed systems which worked prior. The new wire does not have to be connected to interfere, proximity can be enough.
Have a read through chapters 2 and 3 of AC43.13-2B, and you'll have a greater understanding of how to proceed.
https://www.faa.gov/documentLibrary/...2043.13-2B.pdf