Low note problem

I have been playing the alto for nine months and can get good low notes (below low D) to sound quite nicely - except that recently every time I tongue a low C or lower, it sounds for a split second an octave higher, then falls to the correct note. If it is a short note it hardly has time to sound correctly before I'm moving on. If I am running down the scale with tied notes they sound fine (at least, they are on the correct pitch!).

I have tried to change my embouchure and air supply, and different reeds, but can't cure it.

Any ideas?