Sapphire RX Vega 56 sold by NewEgg.com on eBay w/free shipping: $419

I have 2 such cards. They can't work normally with 950 MHz HBM overclock... Should I maybe rise HBM voltage?..

Did you follow the guide there?

https://www.reddit.com/r/MoneroMining/comments/74hjqn/monero_and_vega_the_definitive_guide/?limit=500

I got the same issue at first, then I realize it is because I connect my monitor with this vega card. I use another motherboard with intel igpu and everything works great since then.

That's right! You have to connect the monitor to the IGPU. If you connect it to the Vega's then it's unstable and the system crash.