Hmm. Well it sounds like your system is more stable at the lower clocks, just like ours, only in your case the effects are more pronounced. If I were you I'd try replacing the power supply with a heavy duty 700W from a reputable brand like Antec or Corsair as was suggested earlier. I intend to do the same myself. And if the price currently makes that impossible, maybe try swapping in a buddy's heavy duty power supply and see if that works?
I think the psu is going to be the next route I investigate simply because we've now ruled out so many other things (overheating, drivers, vbios versions, etc. etc.).
Apple Cinema Display A1081 20" 1680 x 1050 (60 Hz)
Windows 7 Home Premium x64
ASUS M4A785TD-V EVO Mobo
AMD Phenom II X4 965 @ 3.4 GHZ (+ Hyper 212 Evo HSF)
AMD ATI Radeon HD 5870 1GB
8 GB DDR3 @ 1333 GSKILL
OCZ ModXStream Pro 600W modular psu