Me too!
Seems to be that this series amps are defective because i've tried in past to change all the rail caps thinking that the original caps are too cheap replacing it with a good nichicon product, but in few days the owner has the same problem.
I've talking about of this "problem" with ZENON CEO, (Mr. Frank) and he sais that a test that i can do is increasing a TANTALUM CAP on the PS driver board that manage the SOFT START at the start of the amp (he says that increasing this value, maybe i can solve the "problem") but i dont think.
He also says that this is the first time he hear of this problem on this particular model (NC5KSPL) produced for very large number of brand all over the world.
But really, this is a common problem on all the serie, because i've problem also on the bigger models that use "FANGZE" capacitors and use same chipset configuration (PS + AUDIO driver board), generally "ZNCM".
You can find a particular NC5KSPL in this video:
ZENON NC5KSPL - potenza reale [RESISTIVE LOAD @2OHM] - YouTube
As you can see, there are problems in RAIL CAPS or RAIL stabilization, because my signal become clip first in a semiwave and so in the other semiwave, due to the imminent failure of the rail caps.
Furthermore, seems to be that this "problem" occur only when the amp is mounted in a car, with the engine on (running alternator) and specially with a very low or totally zero volume.