I am playing classic client only.
Yes, you can never know for sure what's causing the problem.
I just had very bad lag spikes in the 2D client aswell a few minutes ago and my connection is just fine:
>ping -t drachenfels.owo.com
Pinging central-eu2.owo.com [159.153.156.150] with 32 bytes of data:
Reply from 159.153.156.150: bytes=32 time=28ms TTL=49
Reply from 159.153.156.150: bytes=32 time=30ms TTL=49
Reply from 159.153.156.150: bytes=32 time=29ms TTL=49
Reply from 159.153.156.150: bytes=32 time=30ms TTL=49
Reply from 159.153.156.150: bytes=32 time=29ms TTL=49
Reply from 159.153.156.150: bytes=32 time=29ms TTL=49
Reply from 159.153.156.150: bytes=32 time=30ms TTL=49
Reply from 159.153.156.150: bytes=32 time=28ms TTL=49
Reply from 159.153.156.150: bytes=32 time=28ms TTL=49
Reply from 159.153.156.150: bytes=32 time=28ms TTL=49
Reply from 159.153.156.150: bytes=32 time=28ms TTL=49
Reply from 159.153.156.150: bytes=32 time=30ms TTL=49
Reply from 159.153.156.150: bytes=32 time=29ms TTL=49
Reply from 159.153.156.150: bytes=32 time=30ms TTL=49
Ping statistics for 159.153.156.150:
Packets: Sent = 14, Received = 14, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 28ms, Maximum = 30ms, Average = 29ms