I've been using a 520 for several months, as far as I am aware there has never been a firmware update for the 520 series drives, the current firmware is 400i. There have been updates to the Intel SSD Toolbox, the current version is 3.1.1. And it would be through the toolbox that the firmware updates but again, even after a few releases of the toolbox there have been no firmware updates. My drive has been rock solid reliable.
Not much more to offer other than installing the latest toolbox, make sure you are using a good SATA cable and everything is connected up firmly. If you can get the drive working run the toolbox, run a quick diagnostic scan, and of course you should have been running the optimizer function weekly.