Ping takes a very long time to start

Question

The internet on my mac keeps slowing down and I'm struggling to diagnose the problem. Other identical machines on the same network aren't suffering from the same problem. After a reboot the internet (and pings) is all quick and normal, but after a few hours or days things start to crawl. When pinging an address (using network utility) it takes a good 10-15 seconds for the first response. When it eventually gets started, all the response times are very good (even the first one!). It just takes a long time to initiate. What could cause this, and how could I go about diagnosing things further?

I'm connected via Wi-Fi and the transmit rate is always a consistent 300mbps.

Ping has started…

[10-15 second delay...]

PING zdnet.com (50.112.160.88): 56 data bytes
64 bytes from 50.112.160.88: icmp_seq=0 ttl=50 time=173.677 ms
64 bytes from 50.112.160.88: icmp_seq=1 ttl=50 time=173.075 ms
64 bytes from 50.112.160.88: icmp_seq=2 ttl=50 time=173.530 ms
64 bytes from 50.112.160.88: icmp_seq=3 ttl=50 time=172.780 ms
64 bytes from 50.112.160.88: icmp_seq=4 ttl=50 time=176.745 ms
64 bytes from 50.112.160.88: icmp_seq=5 ttl=50 time=173.354 ms

Additional details:

I've noticed that if a webpage is taking a very long time to load and I flush the DNS cache, some or all of the page then loads normally, but any subsequent page loads still takes ages. If I keep flushing the cache ever 2 seconds then the things load okay. Very odd!