Randomly not firing, full battery, not in stand by

Hi,

I bought a second hand Nikon SB-600 and once at time he don't flash. It is not in stand by, the battery are full. I tried on the camera and off camera with Wave 2 triggers.

From 10 tries 3 or 4 he are not firing. When this occurs, the red led which indicate the "charged" status is blinking, and the lamp not firing.

I cleaned all the contacts, both on the flash and the camera. Now the contacts are perfectly clean.

Do you have any idea what can be and how can I fix this?

Thank you so much!