Problème de latence depuis une semaine

Bonjour.

Depuis une semaine, j'ai des problèmes de latence sur mon réseau (wifi et câblé).
Lorsque je ping le modem, tout va bien.
Lorsque je ping une adresse internet, j'ai par moment un temps de réponse anormalement long :

"Date","Time","Address","Min","Max","Ave"
"ven. 22/07/2016","17:17:18,09","www.google.be",12,43,22
"ven. 22/07/2016","17:17:21,14","www.google.be",13,17,16
"ven. 22/07/2016","17:17:24,21","www.google.be",18,1402,366
"ven. 22/07/2016","17:17:28,63","www.google.be",14,26,19
"ven. 22/07/2016","17:17:31,67","www.google.be",12,23,17
"ven. 22/07/2016","17:17:34,71","www.google.be",12,23,16
"ven. 22/07/2016","17:17:37,78","www.google.be",16,2052,525
"ven. 22/07/2016","17:17:41,86","www.google.be",14,23,17
"ven. 22/07/2016","17:17:44,89","www.google.be",12,16,14
"ven. 22/07/2016","17:17:47,92","www.google.be",15,2042,525
"ven. 22/07/2016","17:17:52,97","www.google.be",12,34,22
"ven. 22/07/2016","17:17:56,02","www.google.be",12,26,18
"ven. 22/07/2016","17:17:59,07","www.google.be",13,25,16
"ven. 22/07/2016","17:18:02,10","www.google.be",12,2085,532
"ven. 22/07/2016","17:18:06,21","www.google.be",12,16,13
"ven. 22/07/2016","17:18:09,26","www.google.be",11,17,13
"ven. 22/07/2016","17:18:12,29","www.google.be",14,2137,546
"ven. 22/07/2016","17:18:17,44","www.google.be",12,43,19
"ven. 22/07/2016","17:18:20,46","www.google.be",12,31,19
"ven. 22/07/2016","17:18:23,49","www.google.be",12,35,18
"ven. 22/07/2016","17:18:26,59","www.google.be",13,1937,495
"ven. 22/07/2016","17:18:30,56","www.google.be",17,34,21
"ven. 22/07/2016","17:18:33,61","www.google.be",12,23,16
"ven. 22/07/2016","17:18:36,64","www.google.be",15,1961,510
"ven. 22/07/2016","17:18:41,63","www.google.be",14,25,20
"ven. 22/07/2016","17:18:44,68","www.google.be",13,18,15
"ven. 22/07/2016","17:18:47,71","www.google.be",13,21,15
"ven. 22/07/2016","17:18:50,74","www.google.be",12,1982,505
"ven. 22/07/2016","17:18:54,75","www.google.be",11,23,19
"ven. 22/07/2016","17:18:57,81","www.google.be",12,27,17
"ven. 22/07/2016","17:19:00,84","www.google.be",14,31,19
"ven. 22/07/2016","17:19:03,89","www.google.be",16,21,17
"ven. 22/07/2016","17:19:06,94","www.google.be",14,19,16
"ven. 22/07/2016","17:19:10,02","www.google.be",13,17,14
"ven. 22/07/2016","17:19:13,12","www.google.be",13,1849,480
"ven. 22/07/2016","17:19:17,00","www.google.be",13,23,17
"ven. 22/07/2016","17:19:20,02","www.google.be",13,23,17
"ven. 22/07/2016","17:19:23,06","www.google.be",12,2122,541
"ven. 22/07/2016","17:19:28,22","www.google.be",13,34,22
"ven. 22/07/2016","17:19:31,25","www.google.be",12,36,20
"ven. 22/07/2016","17:19:34,28","www.google.be",12,50,25
"ven. 22/07/2016","17:19:37,32","www.google.be",13,2128,546
"ven. 22/07/2016","17:19:41,52","www.google.be",14,22,17
"ven. 22/07/2016","17:19:44,55","www.google.be",11,16,13
"ven. 22/07/2016","17:19:47,58","www.google.be",13,2002,513
"ven. 22/07/2016","17:19:52,60","www.google.be",12,14,12
"ven. 22/07/2016","17:19:55,63","www.google.be",14,34,20
"ven. 22/07/2016","17:19:58,68","www.google.be",13,31,19
"ven. 22/07/2016","17:20:01,71","www.google.be",13,2122,543
"ven. 22/07/2016","17:20:05,86","www.google.be",13,15,13
"ven. 22/07/2016","17:20:08,88","www.google.be",13,18,14
"ven. 22/07/2016","17:20:11,94","www.google.be",16,2135,548
"ven. 22/07/2016","17:20:17,09","www.google.be",12,21,15
"ven. 22/07/2016","17:20:20,12","www.google.be",13,26,17
"ven. 22/07/2016","17:20:23,16","www.google.be",14,17,15
"ven. 22/07/2016","17:20:26,22","www.google.be",13,24,17
"ven. 22/07/2016","17:20:29,25","www.google.be",14,25,18
"ven. 22/07/2016","17:20:32,28","www.google.be",12,21,17
"ven. 22/07/2016","17:20:35,31","www.google.be",12,1989,514
"ven. 22/07/2016","17:20:39,36","www.google.be",11,51,26
"ven. 22/07/2016","17:20:42,39","www.google.be",14,21,17
"ven. 22/07/2016","17:20:45,46","www.google.be",12,22,15
"ven. 22/07/2016","17:20:48,49","www.google.be",14,2059,540
"ven. 22/07/2016","17:20:52,59","www.google.be",12,19,14
"ven. 22/07/2016","17:20:55,61","www.google.be",13,17,14
"ven. 22/07/2016","17:20:58,70","www.google.be",13,2045,523
"ven. 22/07/2016","17:21:03,76","www.google.be",12,29,20

J'ai quelques redirections de ports définis mais qui existent depuis des années et n'ont jamais posé problème avant ça.
Lorsque je fais un tracert, la latence apparaît aléatoirement, donc ce n'est pas un noeud spécifique qui semble poser problème.

Je suis sur une IP fixe, avec les serveurs dns suivants :
62.197.111.140
109.88.203.3

J'ai déjà redémarré le modem, sans changement.

Quelqu'un aurait une idée du problème?

Merci d'avance pour votre aide.