Would anyone know why the graphs have 30 minute gaps every few hours? CPU becomes a broken line as if it were erased (is averaging 2 percent); bits and blocks drops to zero.

Is only running a mailing program, and it seems to be running ok (no stopping/starting).

That is not indicative of a problem on your Linode. Gaps in your graphs are due to the monitoring service not connecting to the host during those times. This is likely an intermittent routing issue. There is no reason to be concerned about the gaps.