Shift/skip every 1 1/2 revolutioins

[LeeS773]

My
Next,
Breakpoint Pro,
21 speed

is suddenly, on every 1 1/2 rotations of the pedals, trying to shift gears all by itself and then does a short skip..

It happens in every gear.

Where do I begin to look?

[xerxes]

Sound like it could be a stiff link in the chain, try this: http://bikeride.com/stiff-chain-link/