ULTRA FLANGER. Helppp!

[rhodes]

     Hi. Can anyone help me with this?
     
     I built John Hollis' Ultra Flanger project. It's quite cool and is working. My only problem is that the cone of my speaker violently moves up and down, even without input, when you turn down the manual pot of the flanger and volume of amp at moderate high level. I think this oscillation is coming from the multivibrator circuit of the flanger, if i'm not wrong. How do I isolate the oscillation? Will high pass filters at the output stage work? Will the low frequency scillation damage my speaker?   

     Thanks.