Several months ago I replaced my double with a triple biopace crank. For these several months I have only ridden in the small and middle chain ring with no problem. Yesterday I went to a place to ride where I could use the big chain ring. It didn't take long for me to find a problem. It seems that when in the big ring and cross chaining only one or two cogs the chain wants to shift back and forth between the middle and large ring. It does this not on every pedal revolution but maybe one out of 10 revolutions. The shift is very quick like 1/2 crank or less.

To minimize this automatic shifting I let the deraileur rub on the chain a little. So it seems to me that the shape of the biopace chain ring works like a great shifting ramp.

Have you guys ever noticed this with a biopace chain ring or am I doing something wrong which is causing this.