You might have a wiring location problem. If a wire, even not at all related to the avionics, is run close to comm and GPS antennas, there can be interference of the GPS caused by the comm. During flight tests for EMI, I test for this, and have found it. The following is an extract from my test plan. You can perform this test yourself on the ground (considering frequency allocation of local ATC)
5.2 Test Frequencies
When testing VHF Comms as a Source, the test frequencies shown below will be selected when assessing the GPSs as the Victim.
121.150, 121.175, 121.185, 121.190, 121.200, 131.250, 130.285, 131.275, 131.290 131.300 MHz.
The lack of interference from VHF radios will be demonstrated on the completed GNSS installation by tuning each VHF transmitter to the frequencies listed below and transmitting for a period of 35 seconds, or the longest uninterrupted transmission possible for that transmitter, while observing the signal status of each satellite being received. Degradation of individually received satellite signals below a point where navigation is no longer possible is not acceptable
Test each of the frequencies. I have seem all the satellite signal strength bars drop right off when the comm was transmitted on just one of those frequencies. They are specifically chosen, as they are a harmonic of the GPS frequencies.
The organization who did the avionics installation, or any subsequent wiring changes to the aircraft, should be doing this test post installation. If they have a surprised look on their faces when you describe your doing this test to them, you should have a heart to heart chat with them about the completeness of their work.