In most cases this is due to an inadequate antenna, the signals can be screened by the aircraft depending on the sattellite position and finally it gives up.
Another cause is your VHF radio which can switch off the GPS antenna loosing the signals completely. Most GPS antennas are "Active" that is, they are a simple antenna with a transistor amplifier built in. They are broadband to strong signals and when presented with a nearby transmitter, the amplifier saturates, and goes from being an analogue amplifier to a digital switch, dissconnecting the antenna from the receiver. Antenna location is important.