bxb9

Always. Certified mail.

Something like:
 

Congressional follow-up is good, maybe premature; I would've left that for when the 120 days allowed by law are up.  
 
I still strongly suggest you send the letter for 2 main reasons. First, you want to reach the immigration officer who's reviewing your case, not administrative staff who may screen out the service requests,  who can simply say (and rightly so) that your case is still within allowed processing time. And second, you want to leave a paper trail showing that you've exhausted your administrative options in the event you have to sue (hopefully it doesn't get to that).
 
Don't be nervous about writing to USCIS. In my experience, people under-communicate, fearing retribution, and it hurts their case. While my case is slightly different than yours, I sent a letter immediately after 90 days were up, and I got a call about 3 weeks later with a definitive resolution to my case.

As you may know, USCIS has 120 days after your interview to issue a decision before you have any recourse. But I suggest you write a polite but firm letter to the Field Office (see website for their mailing address, which is slightly different than the physical one). Lay out your key milestones and dates (LPR date, n400 application date, biometrics, interview). State the outcome of the interview, remind them that it's now more than 90 days after your "initial examination," and ask for a speedy decision in the time allowed by law. Write it to the attention of the interviewing officer, if you have the name, and send it by certified mail.

91 days after interview nothing. Status still shows "Interview was scheduled". USCIS customer service was no help. I passed the test view but was given Decision cannot be made. Anything I should do?

I was interviewed on June 14, decision was not made. I am still waiting for approval for more than 2 and half months. Online status shows "Interview was scheduled.