The "Thuban" Phenom II X6 / X4 chips typically can't go much beyond 4.0GHz. You may be able to squeeze an extra 0.1GHz or so, but don't expect much more.
From experience (I had a Phenom II X6 1090T @ 4.1GHz), you may need to put a bit more voltage to keep it stable at 4.1GHz / 4.2GHz -- very possibly in the 1.45V to 1.5V range.
Also did you just set the multiplier to 20.5 and left everything else on Auto?
If so....that's not how you overclock things...you're doing it wrong. The chip may be capable of 4.1GHz (as told by the previous user), but it will vary a bit depending on things such as the motherboard used.
For an example, the previous user may be using a custom loop liquid cooler, and a ASUS Crosshair IV Formula 890FX motherboard. That is quite a bit different from your ASUS M4A79TD and Corsair H100i.
You will need to adjust some of the other voltages and settings - not just the Core voltage - to keep overclocks stable.
- CPU/NB voltage,
FX-8350 @ 4.8GHz // ASUS Crosshair V Formula 990FX // Sapphire Vapor-X HD 7970 + Dual-X HD 7970 // 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