K8064 only works for few seconds


I’ve assembled a k8064 kit, seemed to work fine at first. but after a wile it stop working.
When i take off the power for a few minutes; it will work for a few seconds. The longer i take of the power the longer it works normally before it stops.
I’ve check for false contacts and checked polarities and such but I have no clue what is wrong?

Help would be greatly appreciated