Is it possible that you lost GPS reception just after you changed COM frequency in the aircraft? There is a know problem with the local receiver oscillators producing rf harmonics which swamp the GPS receiver when certain COM frequencies are set. One such know problem frequency is 119.875 (Scottish FIR) but there are many others. Note that it is the receiver section of the COM box that causes the problem and the problem is worst if using an aerial close to the radio stack.