I have been using IFR aproved GPS uints for the last 7 years or so and have not had it drop off line for more than 10 min in the whole 7 years except for during the Iraq war when it was off line for a whole day ( to civil users )
I am sure that all your problems are due to the GPS signal not getting to the unit as the signal is so week the aerial positon is critical both to see the signal and to be free from harmonic jamming from the VHF comms ( and DME ? ) the other thing that can give trouble is the gas dicharge display transformer inside the radios fitted to the aircraft.
This problem is normaly only found with Narco radios and can be fixed at modest price by Gatwick avionics at Stapleford.