what is WaitOnLock Error

I get this error: WaitOnLock: error on wakeup - Aborting this transaction
What does it mean?

Background:
Running Postgres 6.4 on Linux. I have a process to handle subscription
requests via email. We are running QMail and it directs the mail to this
'c' program I wrote. It parses the message and tries to add the user to the
database.

This is the sequence that occurs when the process fails:
begin work
select from customer .... returns 0 row (not on file).
select nextval('seqcuscustid') returns next custid
insert into customer ( custid, email ) values ( 9999,
'user(at)somedomain(dot)com');
This is where the error occurs

The process works all the time when I test it with single messages. It
fails 75% of the time when I redirect a bunch of messages (These messages
have been accumulating in a Eudora mailbox while I got the process setup).

Might this be some sort of resource problem?

Bryan White
ArcaMax Inc.
Yorktown VA
www.arcamax.com