Bad caps in the supply..
The protection circuit is most likely detecting excessive ripple..
Also the I've found bad caps in the protection circuit itself..
A warm up tends to make some caps perform better. At some
point, it'll get worse.
Also, you may want to check for bad joints.

Though it's possible there /is/ something wrong with the protection circuit,
five seconds is not an unreasonable delay. If the amp sounds okay, and works
correctly... why go poking around?