the speed drops after some time

[svobodnui11]

-O in my practice it increases the speed, but for some reason it does not find the password that I know in advance, since I did tests, it is very strange why this is so.
I did not reduce the voltage, since in my opinion this will reduce the power of the cards and I will get an even lower result, all I did was set the "power limit" from 100 to 90 so that the cards do not go beyond 70 degrees.
Thanks for helping

4090 and undervolt does not necessarily means reducing hashrate. Because modern cards have multiple limits one ot them is max power consumption and if clock speed is high, voltage is high, and combine power is greater than xxxW then it reduces clock speed but not voltage because of stability. But all cards are over engineered and if you reduce only for 50mV, power is slightly less but clock speed remains high. In modern overclocking power down=overclock 

Try it. 
Also, are cards over risers 1x or are on risers fullX Speed?

1x because the full one would not reach the length
OK, i try change power