Audio "freezing" - sfplay issue?

Something very odd, which I've been unable to figure out.

I have a patch which loads a random audio file into an sfplay~. When the file is close to its end, another random file is triggered into a 2nd sfplay, which starts playing as the outputs are xfaded. There's a bunch of other stuff involved, (the selection of which folder of files to choose from is handled by another patch, and the decision sent to the sf player patch), but this all basically works.
Except ....
The first time a folder of audio files is chosen and one of the files is loaded, it plays fine.
Random selection and play of that folder carries on ok.
If I select a different folder, every thing is fine until the first file is about to play, and then audio stops.
According to DSP Status, audio is on, but the activity indicator shows 0.
There's a line~ object controlling the module level. If I send it a message to change level, nothing happens.

If I do one of two things, audio starts working again (and the problem doesn't reoccur - ie, the patch works normally thereafter). If I either turn audio off and on again, or create a new audio object in the patch, suddenly the audio is running again.

So I don't think there's anything wrong with the patch, as it does run normally once this "freezing" has happened once.

What on earth can be going on here? Audio is apparently on, but not running. Creating a new audio rate object clears the problem and it then runs ok. No error messages in the Max window.
Is this something that sfplay~ (or sfinfo~) can somehow cause? I'm very bewildered!

thanks

David