mqueue backing up

[PC]

Every few days I get a backlog of messages in my mqueue.in for 2-3
hours.  During this time frame, message delivery can be delayed for up to 3
hours.  This could be related to influxes of mail, but unless a small
mqueue backlog produces a domino effect similar to the recent power outage,
it would seem strange that they all last for a few hours.  The mail server
if a high volume server pushing around 28k messages/day.  Any ideas other
than a hardware upgrade?

Thanks!

Paul