My gpu overclock is throttling

I'm using an evga gtx 260 and an msi gtx 260 in sli. The evga was stable w/ a base clock of 714, shader of 1534, and a memory clock of 1230. I got those with evga precision tuner. To overclock the msi one, I went into nvidia performance tuning menu, and set the values. but whenever I run 3d mark or windows index, it throttles the evga card down to base clock 681, mem 1099, and a lower shader 9I don't exactly remember). I've tried setting all the values in that nvidia tuner, w/o the evga precision, and still nothing. Moreover, I tried lower overclocks for each card, and the same still happens. I'm running a core i7 920 @3.8ghz and have a 750 watt psu, if that helps. What should I do?

I've tried running rivatuner and it doesn't work for me. I can't set the clock speeds. What do you mean by properly overclocked? Is that the same as stable?

No, I don't mean stable. I man to ask if the overclock is actually being applied to the card. It is not all that uncommon for the clocks to be set, but not actually change. If they remain at stock, despite your settings, the faster card will underclock itself during SLI.