I have xfx rx580 8gb that crashes under load but works normally when you lower memory clock from 2000 mhz to 1800 mhz. Tserver tests doesn't show any errors. Is this sign of failing gpu or memory controller?

Memory problem. Try tserver mem>2000 (2050, 2100..)

This problem is not always the fault of the gpu, could be other things like capacitor, one or more phase fault, etc, but most of the time it's a gpu problem or broken solder.
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measure the phases (VCORE) using an oscilloscope so you will know if the gpu is getting enough power.

Thanks for reply guys I tried with TSERVER up to 2200 MHz, no errors. Also every phase is working so shouldn't be power problem. GPU was probably damaged due to mining for longer period of time. I explained situation to the owner, edited bios to downclock memory to 1700 MHz and it works decently for gaming so he is happy.