I have new information.
It is not problem with risers at all. When I unplug any GPU, rig works perfect. So rig works perfect with 5 GPUs.
Perhaps problem is with voltage or something else.
What should I do?
Every time I have had issues like this it was due to using an insufficient PSU. You can get a very inexpsensive 500-750w PSU + a dual PSU connector and give that a shot. Likely will get you running.
Is it possible? Rig was working perfectly more than 6 months with same PSU. That PSU is on rig right now, no changes.