1000mhz is quiet high... maybe you have reached the cards limits. Every card is different, some can do 1050, some only 900. One of mine only does 950.
Try with normal memory clocks and see if that helps. You can check for artifacts with OCCT or any other tool (They may give different results). If you get any artifact, thats probably the card's limit.
You could also try moving the problem card to the best-vented slot to get it cooler. This also has the advantage that it's not the primary display and crashes may recover easier.
You're right, I put it to 980 and it does quite well.