Phenom II x4 965 won't OC anymore

MSI 790FX-GD70 motherboard, AMD PHenom II x4 965 processor, Corsair H50 liquid cooling, Corsair 850TX 850W PSU, G.SKill Ripjaws 1600Mhz CL7 RAM @ 1.6V, Antec 1200. Sapphire HD 4890 (1000/1130, Furmark stress test stable).

Stock settings

HT Ref Clock: 200Mhz
HT Multiplier: x17
Total Clock Speed: 3.4 Ghz
Voltage: 1.425
Temp on Load (100% on all 4 cores): 40 deg cel.

Overclock Attempt #1

HT Ref Clock: 200Mhz
HT Multiplier: x19
Total Clock Speed: 3.8 Ghz
Voltage: 1.475
Temp on Load (100% on all 4 cores): 46 deg cel.

15 minute Prime 95 test and AMD Overdrive Stability Test are stable @ 46 deg cel.

Now comes the problem. At this point, I increase Voltage to 1.5V, and do ONE of the following:
1) Increase HT Ref Clock to 215Mhz
2) Increase HT Multiplier to x20

Either ways, within 5min of running a stability test, computer restarts. Temperature at time of restart is around 48 deg. cel.

Why is this happening? Phenom II x4 965 is supposed to have a threshold temp of about 62 deg cel. I am nowhere close.

UPDATE:

After making this post, I re-ran a stress test at Overclock Attempt #1 settings, and it ran successfully for 15min at 46 deg. cel.
After the test was completed, I waited for temperatures to come down again. In AMD Overdrive I increased multiplier to x20, increased voltage to 1.5V and hit apply. The changes were made as reflected by CPUZ. At this point, temperature had come down to 38 degrees, each core was at 10% load or less and I hadn't started the stability test yet, when the PC crashed and restarted.
NOw, why in God's name would that happen? There was no load, temperature was 38 deg.!

yeah...i was doing the OC'ing through Overdrive. I finally put in the stable Overclock Attempt #1 config. in BIOS. I'll try tweaking from the BIOS now. I really hope I haven't reached the limit yet cause temp-wise, I have a lot of headroom.