I don't know if this is your challenge, but some busy domains (like PayPal for example) use DNS to load balance between several servers. The TTL on each domain resolution is very short, and a subsequent domain resolution request will return a different IP. PayPal uses 4 servers in the rotation last I checked with a TTL of only a couple minutes.
Take a look in "/ip dns cache" and check that domain with the IP and the TTL. If the TTL is a very short amount of time (minutes or seconds), then that may be the case.