From the error documentation:

Code:
12801, 00000, "error signaled in parallel query server %s"
// *Cause: A parallel query server reached an exception condition.
// *Action: Check the following error message for the cause, and consult
//          your error manual for the appropriate action.
// *Comment: This error can be turned off with event 10397, in which
//           case the server's actual error is signaled instead.
So I suspect that it has more to do with O/S type stuff than actual Oracle issues, then again, perhaps not. Are there any other errors in the alert log, or any trace files generated around the time you get theses errors?

You could try setting the event noted to see what the actual error is.

HTH
John