Unlabeled wait event in DBA_HIST_ACTIVE_SESS_HISTORY

Attached image of DBA_HIST_ACTIVE_SESS_HISTORY shows the usual events, but for this SQL_ID I see many "blank" events -- they are sometimes taking up excessive resources (elapsed time), but the EVENT, EVENT_ID, and WAIT_CLASS are NULL. Notice that P3 implies there is some kind of I/O -- single block reads, but "ON CPU" (?) -- but why? It is generally difficult to set up a deeper trace, partly because it's a production environment but mostly because this happens to random SQL_IDs and we can't turn on tracing for everything all the time. How can I figure out what these "ghost" events are???? Thanks.