I just wanted to let you know that I'm no expert in computers at all but I'm learning little by little. I've been reading the forums for a while now and I've read a couple of postings on this forum talking about bad performance out of the crossfire portion of the graphics. Apparently some people have had good luck with it but others have had horrible expierence with it. Maybe the crossfire could possibly be your problem? Just something that entered my mind when reading your post. That's about as much as I can do to help but hopefully you get the problem fixed.
Generally, when you Crossfire/SLi graphics cards of the same family but different model they will default to the lower common settings between the cards e.g. if one card has 2GB and the other has 3GB they will both act as if they each have 2GB, equally the clock speeds will default to that of the slower card.
As to your power supply, I would see about getting a power supply which is at least capable of connecting 4off 6-pin PCIe cables, a certified power supply is thoroughly recommended. Using the 4-pin molex to 6-pin PCIe adapters is a recipe for disaster. At most I would use 1, even then I would still be looking to supply each arm of the adapter from a different hard drive feed (not different plugs from the same cable).
The PCIe 6-pin cable is designed to transport 75W of power through 3off +12V supplies and 3off ground returns. This means about 2A on each wire, or a combined total of 6A. Trying to channel this much current through the single +12V wire that the hard drive supply has causes a large voltage drop, the wire was only designed to carry about 4A max. The resultant voltage drop causes fun and games with your graphics cards because you cannot supply it with enough voltage or current.
Thank you so much Pauls, this is both enlightening and heartbreaking. But its good to get some closure on this, Ive been researching it for ages. Would you be able to reccomend a replacement? I am tight on money so I will try to sell the PSU and get a replacement with as little supplemental cash as possible.
I could sell the 2 gpus and get a single 2gb card instead? Can someone recommend one?
I am sick of the issues crossfire causes. Especially in Skyrim!!!
I think you are right, any suggestions? which of the new GTX series would be able to max out all or most games, yet inside my budget of say, €300/$380? AND can be powered without problem by my 620w antec PSU?