Canon 430 EX flash problem

Question

The flash did not work perfectly.
No matter you set it to fully manual at your Canon 40D or
to P mode the outcomes were not stable.
the distance was around 2 to 4 meters and the manual setting was
F5.6, 1/60 secs and the ISO was set to auto.
According the shooting record, Canon set the ISO to 400.
- Some with perfect exposures
- Some terribly under exposed
- Some terribyly over exposed.
Can you help please?
Albechao