As frustrating as the wait is, don't give up on those long forgotten interviews and applications. As an example, today I received an email regarding a job I applied to in mid-November 2016! I read that I've been in the running for the position for the last 4 and a half months. I didn't get the job, but my application has been moving around in the system.
Just because time has lapsed, don't think you are out of the race. Remember that it's not uncommon for companies to have a process where applications get backed up for weeks before the hiring manager gets a list of candidates, and there's another delay after the interviews
If you must send a second follow-up letter, don't make it about you, be topical and mentioned something you've read in the news about the company's latest movements or success story. Most of all, hang in there.