Did this problem occur after a storm or power outage?
Also, when it shuts down is the power light blinking at all?
This problem is going on for a while. But somehow it starts working ok again. It stops working since yesterday again. There was no strom or power outage.
When it shuts down, the power light does not blink.
Sorry to hear that, unfortunately this indicates a failure in the power supply circuit of the Yamaha which will require component replacement to fix. The isse is going to be either one or more failed capacitors, regulator IC's, or the Power Supply itself. There aren't any buttons, or resets you can perform to fix this problem though, and the unit will require service. Average cost for this type of repair is going to be between $150-$175 parts and labor, which may or may not be cost effective for you to fix (As a new receiver costs barely more than this). If you like the receiver though and would like to have it fixed, you can use the link below to locate a tech in your area:
When you disconnected the speakers, then you bypassed this possibility of a problem.
When the unit shut down prior, was it hot to the touch?
Also, what do the speakers you're connecting to it say on the back of them as far as impedance (4 ohms, 6 ohms, 8 ohms)?
The unit was never hot to touch.
I have new "JAMO" speakers (total five of them). Speakers are mounted so I can't see on there back. The booklet of the speaker does not say that.
The speaker impedance was not the problem then if the unit was never hot to the touch, and Jamo speakers are well within the range of the Yamaha. The issues I stated above- that the power supply section of the receiver has a failure.