Hi there!
I have a mikrotik routerboard RB435G https://mikrotik.com/product/RB435G connected to 3 mANT19S https://mikrotik.com/product/MTAS-5G-19D120 but when many users get connected to my wireless network my routerboard loses conection for a little time and inmediatly reboots itself. I think my bandwith isn't the problem because there are not many users and and the routerboard can support that trafic (only uses 30% cpu). Also i made tests with each wireless antenna and when only one works my network runs without problems, but when i enable other one the routerboard reboots.

I measured the voltage but it's not the problem either

Nothing to do with voltage, can your power supply deliver ample current? (amps)

Which Mini-PCI card do you use?
Did you overclock the RB435?

Thanks guys! i resolved my problem changing the PoE conector. The current charger supports more voltage.