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Something weird just happened. Using EVGA precision, I boosted the power target a little bit to see if I could speed up the card that is being throttled back. It went up to 90 deg, so I changed the fan setting to aggressive. The fans ran fast for a minute and then all of a sudden they slowed way down, GPU temperatures went down to under 50 deg. and the GPU clock speeds on the GTX 780's went down to about half speed (574 Mhz). Rendering seemed to be stuck, so after a while I cancelled and started rendering again.

However, it is now rendering but about 30% slower, and the GPU clock speeds for both GTX 780's are both stuck at 574 Mhz, with no variation. They are of course running cooler than usual at 65 Deg.
The GTX 750Ti continues to run at around 1200 mhz.

How do I unlock the speed gating of the two 780s?
I tried resetting Precision to the Default setting and even rebooted my PC, but to no avail.

Your cards just fail - its is not a slowdown. Don't overclock them on air, you'll burn them.
To do so you need water cooling and higher voltage.

I called EVGA and was pleased to get someone without waiting (plus they provide 24 hr support).
He didn't know why the GPU speed was being locked at half speed. He suggested trying some benchmarking software and also taking the cards out and then testing them with one installed at a time. I thanked him and did the following:

Installed EVGA's benchmarking sw.It didn't provide any insights.
Uninstalled PrecisionX and re-installed it. GPU speeds still stuck.
Rebooted. No luck.
Shut down and restart. No luck.
Shut down, took out the two 780's and left in the 750Ti. Rebooted and tested the card. It continued to run fine as expected.
Shut down, reinstalled the two 780's and then tested it. They are working at full speed again!!

I learned my lesson not to tweak the settings on air. It seems like some safety setting was triggered when the one card hit 90 deg (wasn't there for long), which cut the two 780s back to half speed. I couldn't have burned them as they are running fine and one of the cards only got to 80 deg. The only way I could get it to reset was to make it forget the cards by running the PC without them.

Until I get water cooling installed I'm going to leave it as is (defaults). Even if I ramp the fans up with the aggressive setting (without touching the GPU settings), the 10% improvement in the speed of the one card is only cutting render times by one second.

To test whether the higher temps on the one 780 are due to its position or a variation in the card performances, I swapped them.
The result is that it is the position that made the difference. Whichever card is at the top runs hotter.

Hopefully this thread will be of use should anyone else encounter the same anomaly!

If you are on air your card even at stock clocks may fail - heat causes it to fail. The only remedy for this is then to reboot.

If you are on water - temps are 25-45C on gpus and then you may go with 1300mhz or even higher with higher voltage (1.2 is safe; higher volt requires hacked bios, I would not do it for octane purposes).

And here is a tip for anyone who is on air and have high temps: If you are upstairs on top floor (loft, office in attic etc), point a semi-powerful fan down the steps. Do this year round, regardless if you have central air/heat. You will likely see your temps drop some.. Every little bit helps. I have a loft style condo and office is in loft. Even though I'm water-cooled, I see a difference of +5°F or more (GPU temps) when fan is off...

FrankPooleFloating wrote:And here is a tip for anyone who is on air and have high temps: If you are upstairs on top floor (loft, office in attic etc), point a semi-powerful fan down the steps. Do this year round, regardless if you have central air/heat. You will likely see your temps drop some.. Every little bit helps. I have a loft style condo and office is in loft. Even though I'm water-cooled, I see a difference of +5°F or more (GPU temps) when fan is off...

Yeah.. I had 4 of those hot 580's as well
I might start a new thread (so not hitch this one) about exchanging ideas how to make a small (semi?)closed space for pc(s)
and exhuast the hot air from the office room to outside asap instead of heating the office space too much in summer.
Greetz,