As black_zion stated, you NEED to swap out your PSU for something more reliable and higher in quality.
1.56v Vcore seems a bit high. From what I can see on the interwebs, for speeds under 5GHz 1.3v to 1.45 / 1.47 / ~1.5v is all is needed.
Then again, each CPU is different - but like any overclocking, if lower voltage is possible, go with it.
Personally, I don't feel safe using lower-end boards for higher overclocks. They have fewer power phase / power delivery components.
For your board right now, it uses a 4+2 phase design VS the 6+2 on ASUS's 990X chipset boards VS 8+2 on the 990FX based boards.
The higher-end boards also use higher-quality components and improved cooling solutions.
Overclock one part at a time. CPU then GPU or vise-versa.
I'm sure the 4850's can go a bit further than 20MHz/7MHz. What cards are you using? (brand & model)
When was the last time you had them dusted?
They are a couple years old now, and the warranty should be over, disassemble the cooler, clean off old thermal paste, and apply a new amount. (I did this on my 4850 last year...I got photos if you need)
Mess with one HD 4850 at a time (one in your system at a time); maybe one of your two just doesn't like being overclocked.
Phenom II X6 1090T @ 4.1GHz (& FX-8350) // ASUS Crosshair V Formula 990FX // Sapphire Radeon HD Dual-X 7970 @ 1150/1500 // Thermaltake Frio w/ push-pull (using Antec Formula 7 Nano-Diamond + Cooler Master JetFlo fans) // 8GB (2 X 4GB) G.Skill RipJawsX 2133MHz // Corsair TX850 850W // Corsair Force GT 120GB SSD (OS) // Kingston V200+ 120GB SSD // WD Caviar Black 1TB // Windows 7 Ult x64
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