Unstable When I Underclock!

[diabinc141]

During the day I run these settings on my 5870 + 6870 Rig:

5870 - 900/300 @ stock voltage (67C) 50% fan - 403.1 MHash/s
6870 - 950/300 @ stock voltage (58C) 45% fan - 299.0 MHash/s

But at night it is very loud as I sleep in the same room as the rig so I am trying to lower it to these settings:

5870 - 700/300 @ 1.000V (57C) 30% fan - 312.6 MHash/s
6870 - 750/300 @ 1.000V (54C) 25% fan - 237.5 MHash/s

It runs stable for about 3 minutes then GPU load goes down to 0-30% for about 30 seconds then back up to 99% for a while then wildly fluctuates. It keeps doing this on both cards and at the same time. I have tried to raise the voltage (which doesn't help), but that kind of defeats the purpose as I want to keep it as quiet as possible and not raise fan speeds. Ironically I can get them to run stable far above their stock speeds but not lower. Is there something I'm missing? Is this an obvious thing that I'm overlooking? Any help is greatly appreciated!

[1714182037]

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[1714182037]

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[Kluge]

Try again with mem speed @ 600. Different software sometimes reports the number differently. It's possible you're running the mem speed at what would commonly be assumed is 150MHz.

May also be auto-clocking "features" enabled, perhaps in AMD's CCC?

[P4man]

Its not the clock, its the voltage. default voltage for most 5870s is 1.175v. 1v is a huge 18% undervolt.
Or are you saying 700 Mhz doesnt work 1.175v?

[Kluge]

Its not the clock, its the voltage. default voltage for most 5870s is 1.175v. 1v is a huge 18% undervolt.
Or are you saying 700 Mhz doesnt work 1.175v?

I have tried to raise the voltage (which doesn't help)...

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Yeah but he doesnt say how far.
 Many cards cant be set any voltage but only to some predefined values.  If his cards are not stable at 700 stock voltage he should state that and not post results for 700/1v.

Good point.

[P4man]

Yeah but he doesnt say how far.
 Many cards cant be set any voltage but only to some predefined values.  If his cards are not stable at 700 stock voltage he should state that and not post results for 700/1v.

[diabinc141]

Sorry for not being clear but yes I'm sure it's not stable at 1.175V. I'm using MSI Afterburner and I have no problem over clocking either card but anything under the stock speeds isn't stable.