I agree with your proposed settings, but I’m confused about your ‘no-audio’ comment. On the failing calls, do you hear a typical choppy voice problem (gaps of a fraction of a second)? Or does audio stop for more than one second at a time? Or does it go completely silent?
To start, the simplest test is to record the calls with the regular FreePBX record features and hear whether the recording has the problem.
There is a relatively new tool for capturing the raw packets on a per-call basis, which you can then analyze with Wireshark, http://pcapsipdump.sourceforge.net/ .
On my own system, I run
tcpdump -s 0 -C 100 -W 100 -w rbuf -Z root &
which captures all packets into a ring buffer of 100 files of 100 MB each. This runs 24/7. When trouble is reported, I look at the timestamps of the files to find the one containing the bad call. Sometimes I’m unlucky and the call overlaps two files (which can be merged in Wireshark). Then, I use Wireshark commands to analyze the specific call. In some cases, having the raw stream is useful; you can see what else was going on at the same time.