Macbook Pro finder error -50 appearing randomly
Since it's out of warranty your options are a bit limited. I would start by bringing to the Apple store for diagnostic, or read here for how to get your own hardware diags.  You have already reinstalled a fresh OS on a new drive, so it's possible the SATA cable is bad (believe it or not I've seen this happen), or even the logic board.  

What errors are reported in Console log when the problem occurs?