Wireless microphones work by transmitting high frequency signals to a receiver. If your wireless microphone is cutting in and out, there are a few troubleshooting techniques you can use to get it working correctly.
Cable
If you are using a lavalier, make sure the cable connecting the microphone to the belt pack is dangling down and not twisted.

Belt Pack Position
Make sure the belt pack for your wireless lavalier is attached to your pants or belt at your back.

Batteries
Change the batteries and put in new batteries.

Change Frequencies
Change the frequency settings on the microphone and the receiver until the microphone works correctly. There may be other devices in the area using the same frequency you have been using causing interference with your unit.

Change Tracks
If you are using a soundboard, change the track or channel into which the receiver is connected.