...さっきまで200msほどかかってたけどようやくまともになってきた。

$ ping www.yahoo.co.jp
PING www.yahoo.co.jp (211.14.15.5) 56(84) bytes of data.
64 bytes from 211.14.15.5: icmp_seq=1 ttl=234 time=26.7 ms
64 bytes from 211.14.15.5: icmp_seq=2 ttl=234 time=72.9 ms
64 bytes from 211.14.15.5: icmp_seq=3 ttl=234 time=26.0 ms
64 bytes from 211.14.15.5: icmp_seq=4 ttl=234 time=228 ms
64 bytes from 211.14.15.5: icmp_seq=5 ttl=234 time=44.1 ms
64 bytes from 211.14.15.5: icmp_seq=6 ttl=234 time=26.2 ms
64 bytes from 211.14.15.5: icmp_seq=7 ttl=234 time=134 ms
64 bytes from 211.14.15.5: icmp_seq=8 ttl=234 time=27.0 ms
64 bytes from 211.14.15.5: icmp_seq=9 ttl=234 time=26.1 ms
64 bytes from 211.14.15.5: icmp_seq=10 ttl=234 time=105 ms

10.0.31.129 というルータが犯人か?
# traceroute -f 15 www.yahoo.co.jp
traceroute: Warning: www.yahoo.co.jp has multiple addresses; using 210.81.150.5
traceroute: Warning: Multiple interfaces found; using 218.130.208.2 @ rtls1
traceroute to www.yahoo.co.jp (210.81.150.5), 64 hops max, 40 byte packets
15 10.1.0.249 (10.1.0.249) 10.778 ms 10.893 ms 10.694 ms
16 10.0.31.129 (10.0.31.129) 196.081 ms 198.310 ms 198.119 ms
17 10.9.17.137 (10.9.17.137) 199.186 ms 199.364 ms 200.093 ms
18 YahooBB218176246002.bbtec.net (218.176.246.2) 201.690 ms 200.940 ms 198.900 ms
19 210.81.152.78 (210.81.152.78) 197.995 ms 197.122 ms 196.410 ms
20 *^C