In guiminer, one GPU in my 6990 works fine, the other has connection problems

[super6]

I thought it might be a power issue but it wasn't. When I start both GPU's up they start fine then, when they hit about 60% usage the 2nd one gets "connection problems" and drops back to 0. This isn't a username/password issue.

[gigabytecoin]

I thought it might be a power issue but it wasn't. When I start both GPU's up they start fine then, when they hit about 60% usage the 2nd one gets "connection problems" and drops back to 0. This isn't a username/password issue.

How can you be so sure it is not a power problem?

How much power do you have dedicated to your 6990s? What PSU are you using exactly?

The fact that the problems start whenever the cards go over a certain use percentage almost tells me exactly that power is the real source of the problem. Your cards draw more electricity as they ramp up in terms of "usage percentage". ie when your card is using 100% usage you are probably pulling ~300 watts each and when you are down to 50% usage you could only be doing 150 watts.

[Jack of Diamonds]

Usage drop to 0% indicates likely thermal throttling.

What is the temperature on the other GPU?

[super6]

I thought it might be a power issue but it wasn't. When I start both GPU's up they start fine then, when they hit about 60% usage the 2nd one gets "connection problems" and drops back to 0. This isn't a username/password issue.

How can you be so sure it is not a power problem?

How much power do you have dedicated to your 6990s? What PSU are you using exactly?

The fact that the problems start whenever the cards go over a certain use percentage almost tells me exactly that power is the real source of the problem. Your cards draw more electricity as they ramp up in terms of "usage percentage". ie when your card is using 100% usage you are probably pulling ~300 watts each and when you are down to 50% usage you could only be doing 150 watts.

Well I had a 670W PSU connected and thought that was the problem so I got a 1100W in but the problem persists. I have another system with 2 6990's (vs. 1 6990 in the trouble system) and it runs fine on a 1500W so I would think that 1100W would sufficient for a single 6990. I agree that it sounds like a power issue but I jut don't see how that could be the case now that I've tested with a new PSU.

Also, I should add that, while the summary page lists connection problems the miner's tab says "Verification failed, check hardware!"

[super6]

Usage drop to 0% indicates likely thermal throttling.

What is the temperature on the other GPU?

74C on the active GPU, 43C on the innactive one. On my dual 6990 system all GPUs are running hotter than 74 so I don't think that's the issue.

[padrino]

Also, I should add that, while the summary page lists connection problems the miner's tab says "Verification failed, check hardware!"

That indicates a hardware problem, assuming with your other rig working you understand the basics I would assume this 6990 has a bad core or VRMs.