The same behaviour I have described here: https://bitcointalk.org/index.php?topic=1165628.msg17518037#msg17518037

It looks like your secon fan's speed is significantly lower. Check if both fans are starting to spin at S7's startup. I guess there will be only one fan spinning, and second will start to spin only because of the airflow of another fan. Btw, check Issue #9 here.

So it looks that your I/O board is broken (the PCB to which the fans are connected). You can try to flash your S7 with 1fan firmware (try to flash it to SD card and boot your S7 with this SD card - if something goes wrong you will be able to just exctract an SD card (when the power is turned off!!!)). In case it will work properly for a while, you can power your second fan directly from PSU.