problem on a MX sendmail machine

[RJ45]

hello I have a FreeBSD machine

FreeBSD infngw.infn.it 7.0-RELEASE-p2 FreeBSD

being a MX record with high loads.
I often ahve this message on logs right before the machien locks up.
any hints ?
thank you

swap_pager: indefinite wait buffer: bufobj: 0, blkno: 482607, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 469743, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 468639, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 500495, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 487647, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 478575, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 510111, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 490815, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 522943, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 513311, size: 61440
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 519727, size: 61440