Hash rates are dropping after the first few minutes of a rig running

[JayDDee]

Those threads are old but the cards are the same, and the problem too,

Different problem, fail to boot or fail to start is not the same as reduced hashrate.

If you are correct it means the 480 requires a dummy plug while the 470 does not.
That's too much of a stretch without some kind of proof. Also why is this problem
only happening now? Both cards have been around for a few years.

"Sleep mode" is just speculation and isn't logical. Lack of a connected monitor is not a
valid reason to put the GPU to sleep.

I don't want to argue with you, I just don't want OP lead down the wrong path.

[rdluffy]

Those threads are old but the cards are the same, and the problem too,

Different problem, fail to boot or fail to start is not the same as reduced hashrate.

If you are correct it means the 480 requires a dummy plug while the 470 does not.
That's too much of a stretch without some kind of proof. Also why is this problem
only happening now? Both cards have been around for a few years.

"Sleep mode" is just speculation and isn't logical. Lack of a connected monitor is not a
valid reason to put the GPU to sleep.

I don't want to argue with you, I just don't want OP lead down the wrong path.

Oh, be cool, we are not arguing, just talking, but OP said he tried to attach a monitor and the card hashes correctly
I've mining for a couple of years, and with RX 470, 480, 570 and 580 we needed this plug to rig perform 100%

You can look for old threads, I'm really shocked that nobody here heard about these plugs, in 2017 this was very commom, I'm not crazy guys, I'm just an old miner who solved hundreds of problems with rigs 
I'm not speculating anything, try to explain to my old rigs that these plug are wothless 

And when you face a problem, why not just try options?

[JayDDee]

I've mining for a couple of years, and with RX 470, 480, 570 and 580 we needed this plug to rig perform 100%

Have you seen these exact symptoms where GPUs with no attached monitor (or plug) start mining at speed
then slow down after a while? Was it solved by adding a plug?

If not it's a different problem.

I like a higher level of confidence before spending money to "try options". More testing (as I previously suggested) is free.

Edit: I found some discussion that the AMD drivers need one active monitor to initialize properly.
If headless, or the monitor is plugged into the IGPU or a Nvidia GPU a plug may ne necessary.