We have experienced failures with HH GPSs caused by interference from the radios on certain frequencies. Also, the Garmins are slightly slow - has any one else noticed this? There seems to be about 15 seconds delay over a given positive location. This is not too much of a problem at height, but can be significant at low level.
You should get the installation checked. One of the requirements of an IFR GPS installation is that one checks the GPS performance at specific harmonics of the other avionics settings, especially the DME. A lot of panel mounts have been thrown in with bad wiring and then you can get this kind of problem.