Voice breaks when playing Asterisk announcement

Hello, I’m trying the simplest scenario, i.e. the “Hello world”-like scenario, but using a longer announcement.
After few seconds, the announcement stops for a while, repeatedly.
From the below reported Wireshark RTP flow excerpt, it is possible to see a 5 sec break between pkts 738 and 1007; the same happens periodically.
Note: Asterisk (v. 14.0.1) is installed in a server behind a NAT/Firewall,but I don’t think this is the reason of the issue.
Thanks for the support.

726,15221,20.003000000000611,0.0099019877487249042,-0.12200000000029831,29.199999999999999,"",“OK"
728,15222,19.998999999999796,0.0093456135144423299,-0.12100000000009459,29.199999999999999,”",“OK"
730,15223,20.005999999999403,0.0091365126697523953,-0.12699999999949796,29.783999999999999,”",“OK"
732,15224,20.001000000000204,0.0086279806279056029,-0.12799999999970169,29.199999999999999,”",“OK"
735,15225,19.995999999999185,0.0083387318387124348,-0.12399999999888678,29.783999999999999,”",“OK"
736,15226,19.975000000000364,0.0093800610987701712,-0.098999999999250576,29.783999999999999,”",“OK"
738,15227,20.042999999999665,0.011481307280076116,-0.14199999999891588,29.199999999999999,”",“OK"
1007,15228,5017.7980000000025,0.060638725575227793,-0.94000000000232831,0.58399999999999996,“SET”,“OK"
1008,15229,2.1739999999990687,1.1709738052268344,16.885999999998603,1.1679999999999999,””,“OK"
1010,15230,20.028999999998632,1.0996004424000718,16.856999999999971,1.752,”",“OK"
1012,15231,19.997999999999593,1.0310004147500929,16.859000000000378,2.3359999999999999,”",“OK"
1014,15232,19.979000000002998,0.96787538882802471,16.879999999997381,2.9199999999999999,”",“OK"
1016,15233,20.003999999997177,0.90763317702609669,16.876000000000204,3.504,”",“OK"
1018,15234,19.997999999999593,0.85103110346199107,16.878000000000611,4.0880000000000001,”",“OK"
1019,15235,20.023000000001048,0.79927915949568207,16.854999999999563,4.6719999999999997,”",“OK”

Enable internal timing and specify a valid timing source/

Based on this document https://wiki.asterisk.org/wiki/display/AST/Timing+Interfaces . Beginning with Asterisk 1.6.1, By default, Asterisk will build and load all of the timing interfaces

modules to be used.
Asterisk includes the following timing modules:
res_timing_pthread.so
res_timing_dahdi.so
res_timing_timerfd.so – as of Asterisk 1.6.2
res_timing_kqueue.so – as of Asterisk 11

So I might be wrong but I think that internal timing and valid timing source is done automatically if modules are present

internal_timing was still a configuration option up to at least 1.8. Modules may have to be selected with make menuselect, but I can’t remember the defaults.

The only other thing that causes this sort of effect is using a virtual machine that has been configured for general purpose use., or which is being moved between hosts.

The problem was very much similar to this issue that I found in the mean time:
https://issues.asterisk.org/jira/plugins/servlet/mobile#issue/ASTERISK-26280
(DNS lookups can block channel media paths).

Anyway the problem was solved after modifying the DNS configuration in /etc/resolv.conf.