Welcome in the Forum!
Too less information about your stroke for detailed hints. But three thoughts FWIW:
- You build up some tenison anywhere in your body and your breathing is not as relaxed as it should be. (This seems to be the most common issue...)
- As you suppose, your rhythm of breathing or your breathing pattern doesn't fit your needs. (Have in mind, you must have exhaled enough to get enough fresh air in with a short sip, and as Terry says, you can't inhale too early...)
- What happens if you force yourself to breathe in rhythm only every third or second stroke with stable head while using the snorkl?
Enjoy your strokes and best regards,