protection light coming on denon 991avr

seems to be an intermitant problem but here it is all within the last 2 weeks
991avr can work for days or turn it on and it shuts down witht the power button flashing.
5.1 setup
trouble shooting
1 removed sub (amp shut down)
2 removed left then right front with sub on (amp shut down)
3 removed the center and then the amp stayed on.
4 repeated this process and same results.
5 removed just the center and plugged in a different speaker to the center channel and amp shut down.

so it looks like the center channel might be the problem or maybe the front speakers?
Any idea of what part of the machine needs repair?
going to repeat the process tonight.
thanks

thanks tested the speakers but still happening so off to the shop where it stil is for the past week and 1/2