key lag on Linode running virtually nothing

I'm running a Linode and consuming <5% memory and virtually no CPU yet I'm getting pauses between keys being pressed and my ssh client responding. This can last up to 5 seconds and is intermittent.

I've done what's obvious to me - run top to check resources, shut down anything I don't need etc

Any steer as to how to go about investigating this further would be very welcome.