Hi there miners!

I recently started to mine Ethereum a month ago, but I have a problem and I think maybe you can help me with an advice.
My RIG is an Intel Pentium with 4 PCI-E motherboard, 2x4GB RAM and SATA HDD.
It has 4 Sapphire RX 570 pulse 4 GB with Elpida mem [first is EDW4032BABG (Micron-Elpida) and second H5GC4H24AJR (SK Hynix)].
I have a 450W PSU for system and 600W modified Dell server PSU for the cards.
I have 4 risers: PCE164P-N07 PCI-E 1X TO 16X VER 007 NTH
I modified the BIOS in SRBPolaris with custom timing found on this forum:
777000000000000022AA1C00315A5436A0550F15B68C1506004082007C041420CA8980A9020004C 01712262B612B3715
I raised the mem freq to 2070 to 2120 (depending how much the card was able to handle).
I set the core to 1150 and the voltages to 850.
I am using Claymore 11.2 with only this in the start.bat:
EthDcrMiner64.exe -epool pool_adress -ewal wallet_adress -epsw x -dcri 6
Immediately when i start the miner it gives me some errors in hwmonitor (mostly under 10 errors), but after this it gives me none.
My rig runs at 124.9mh and i never had a single rejected share!
But my problem is this: from time to tine (from 1 to 7 days), the rig either restarts or freezes (simply the desktop remains like it is and i can’t do nothing, even moving the mouse).
Did I undervolt it too much? Did I raised the mem freq to much? It is possible to be the PSU for the system (a no-name 450W cheap one, but worked good before on my desktop)? Are there the risers? What can i do? I would be grateful for any advice!

PS: Can I change something to improve performance? (now that I stopped the RIG i will put the latest Claymore version)
PS2: Can I dualmine (ETH+DEC) with not risking to burn the PSUs? How can I know the power consumption if i don't have a plug power meter?

Thanks!