A:Answer Could be a few things, most of the time it's a simple wiring error changing the impedance to something the amp can't handle, so when the fets start to get hot the amp will go into protection. Ensure you have the proper impedance speaker, hooked up properly to get the impedance the amp can handle, and you should be good.
If it's not going into protection and is just turning off randomly, check your power/ground/remote wiring, something may be loose.
Also, if the speaker isn't in a proper size enclosure or the enclosure is tuned to a frequency the speaker can't handke, this can sometimes cause the speaker to do strange things in terms of it's load on the amp or its power handling. Could be causing it as well.