Constantly crashing RELENG_4_8 server

[H. Wade Minter]

One of my RELENG_4_8 servers is crashing regularly.  I think it's
hardware, but I'm not sure which part is the culprit.  The crash messages
are below - any suggestions?

My googling is pointing to disk, but I'd like a second opinion.

#####

Jun 16 19:50:56 carlton /kernel: panic: pmap_enter: attempted pmap_enter
on 4MB
page
Jun 16 19:50:56 carlton /kernel:
Jun 16 19:50:56 carlton /kernel: syncing disks... 9


Jun 19 21:29:23 carlton /kernel: panic: pmap_enter: attempted pmap_enter
on 4MB
0age
Jun 19 21:29:23 carlton /kernel:
Jun 19 21:29:23 carlton /kernel: syncing disks... 6 1 1 1 1 1 1 1 1 1 1 1
1 1 1
1 1 1 1 1 ^Q
Jun 19 21:29:23 carlton /kernel: Copyright (c) 1992-2003 The FreeBSD
@roject.


Jun 20 11:00:57 carlton /kernel: panic: vput: negative ref cnt
Jun 20 11:00:57 carlton /kernel:
Jun 20 11:00:57 carlton /kernel: syncing disks... Copyright (c) 1992-2003
The Fr