I have just one board, which was modified to use the linear regulator by Drillbit in Sydney. It has been running now for 'ages', like many weeks without any hiccups at an average hash rate of 22.5 GH/sec. It runs at int:53:2:950 using cgminer 3.10.
The estimated BTC that it has generated is 0.65, and cost about 0.625 BTC, so ROI has been achieved in BTC, disregarding the recent upheavals in BTC value, but it still has some time to go.
How long have you been mining it for? That's alot more BTC then I've gotten and it's certainly long enough that statistical fluctuations should have evened out.