Part of the certification of an IFR GPS (always panel mounted) is that certain harmonic-frequency tests are done (by setting the DME etc to certain values) to ensure there isn't interference.
I've had signal loss only twice: once near Italy in 2004, when both GPSs lost reception for about 2 minutes, and once in Italy in 2006 when the KLN94 took ~ 1 hour to acquire the signal following power-up on the ground (while a handheld worked fine).
The latter instance was weird and was followed up with Honeywell, with the help of various setup screen photos I took at the time, and the best opinion was that it got its ephemeris (or whatever it's called) data corrupted in a weird way. It came back at FL160 over the Alps - I told ATC I had lost BRNAV capability and got VOR-VOR routes and vectors. The handheld GPS had no usable DCT feature for airways intersections; something I have since fixed...