Three guesses:
1. You need to replace the HR battery. If it's a Garmin strap, good luck!
2. The strap isn't getting a clear signal. Wash the strap, and try a HRM gel (or spit) under the pads if washing doesn't work.
3. The readings are right, and you may have atrial fibrillation. Go see a cardiologist.
Or maybe 4, the strap needs to be replaced. (Try a Wahoo for easier battery changes!)