OK so let me know if I got this right.
You had the Apevia 900w.
You used it with your SLId video cards.
It blew up from the strain, so you RMAd it.
You took out the SLI and left in 1 card and an XFX 650w PSU.
It worked fine during the RMA process.
You got another Apevia 900w.
You put the new Apevia 900w in the PC and brought the SLI configuration back online.
Now you have an identical situation as when the first PSU blew up, except that now you are having some different problem.
If all of that is true, it does cause me to look at the PSU with a suspicious eye.
I went ahead and read this nice little Apevia 900w Warlock review here:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=92
which gave the PSU a 5.5 / 10 rating and said it should have been listed as an 800w instead of 900w.
It also said that the PSU ran out of spec many times on different lines when heavy amounts of load was put on it.
I am guessing your SLI setup is capable of similarly heavy amounts of draw with 2x 570s.
I don't know, I am kinda biased against PSUs that get serious ratings of 5.5 / 10 and the thing about 1x working with a 650w and 2x not working with a 900w right, it kinda feels like a PSU thing to me right now.
The video card can pull 75w from the board, another 75w from a 6 pin pcie connector, and 100w from an 8 pin connector (25 per 2 wires on both types) so that is up to 250w that the second card can potentially max draw.
With the XFX 650w another +250 could potentially reach to 900w, and this PSU was said in the review to maybe be an 800w, again I can see some potential problems there.
The thing that really sticks a thorn in my side right now is the fact that these problems exist without heavy loads, as I understand it and why it might work sometimes and not others.
If it were indeed a PSU problem, I would expect weirdness at load rather than at idle.
That being said, PSU problems can surface as problems with nearly any other part.
The exact same setup caused the other PSU to literally blow up, that tells me that the strain this thing must be feeling under load must be unimaginable.
I don't know, it kinda feels like the PSU isn't giving the right amount of juice to the cards and therefore they can't run optimally.
Anyway, that is just a stab in the dark with some reasonable grounds.
Old PSU blew up with the same conditions.
The rest of the PC worked just fine with no SLI.
New PSU having problems with the conditions that blew up the same sort of old PSU.
Review that calls it an 800w, fairly.
I think that is reasonable grounds to consider another PSU at least. Something from Enermax or Seasonic in the 950w - 1000w range approx.