[Adrian]

Hola azapa,
Are you sure you are doing the Frenzel correctly? I had this very same problem with the Valsalva, my dives had more curves than a mountain road and by the time I reached 13 meters I was a minute into the dive...
With the Frenzel that all disappeared, just blowing gently I can equalize easily until my failure depth. (Where the pressure doesn't let you use your lungs any more.)