I use Q-rings with Dura Ace 7900 the last 2 months and I noticed that when I have the last gear, for example 23 in 11-23 cassette or 25 in 11-25 with big(53) and small(39) crankset and I push(pedal hard) it tends to change gear from 23 to 21 or 25 to 23.
It doesn't change but I can hear that the chain tries to go down.
I have searched everything and the only expanation that I have took it's because of the oval shape of the Q-rings.
I just want to know if it's actually from the Q-rings and if I can do anything to stop it.

That sounds like you have an issue with the rear derailleur. The shape of the Q-rings has very little effect on the shifting of the rear derailleur, but does have a little more lag when shifting the front derailleur compared to a round ring (for me, it's worth the trade of).

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"Suddenly the thought struck me; my floor is someone elses ceiling" - Nils Ferlin

Check the hanger, too.

Anyone else??

Nope. No one else. It just ain't the rings. If you are waiting for someone to corroborate your hypothesis that it is, you might be waiting a long time...

I've got the 50/34 Q rings and say that you should be looking at the rear der adjustment.

I used mine for a while, and never had trouble with rear shifting. Like everybody else said, check and adjust your derailleur, and make sure your hanger is straight. Maybe the screw that limits the travel, has been affected?

Check the RD. period.

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mySWORKS
myCERVELO
myBMCRM01
myBHultralight

Like I said, it's the rear derailleur. Or the rider, but I kind of doubt that.

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"Suddenly the thought struck me; my floor is someone elses ceiling" - Nils Ferlin

Rear derailleur. It did affect my front shifting. And like above. Not enough to take them off.

Firstly, thank you all

When you say derailleur issue you mean that I must adjust it or I must start looking for a new one?

Assuming that the cassette is correctly assembled, the chain is correctly installed and both are in decent condition, the symptoms you are describing appear to be a rear derailleur/hanger problem, not a chainring problem.

Most derailleurs have more than enough capacity to deal with the minor changes in chain tension created by 'Q'-rings in normal operation. I do not think you need a new derailleur, it could just need to be adjusted better, though it sounds more like a hanger issue to me.

The cassette is correctly assembled.
Cassette and chain are new.

The chain length must be like with standard rings or different?

With "hanger" which exactly component do you mean?

Chain length is adujusted the same as round rings. The hanger is the little piece of metal that attaches to the frame so you can attach the derailleur. Sits in between the RD and the frame.

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My Cervelo R5

small ring x 25 is the problem, or the big ring x 25? Or both?

If shifting is otherwise perfect then it's just the limiter screw.