I had six SXT SA5 without ac technology working as AP and I lost everyone because of lightning in past week. I changed to SXT SA5 ac with same configuration (with nv2 active). But now they are rebooting after few minutes working (about 5 or more). When I change wireless protocol from nv2 to 802.11, they stop reboot.

I already tried RouterOS:
6.36.4;
6.37.2;
6.38rc34;
6.38rc35;

What am I doing wrong?

did you update firmware also (system routerboard)?

Yes, sure.

Nothing more?

check the voltage via system/health.

It might be the case that the supply voltage is dropping too much over long cable runs.

nv2 might be more energy intense than plain 802.11