Server Optimization Help Request - VERY High Sudden Loads

We just upgraded to VBulletin 3.6.5, and are experiencing strange behavior. My forums are on a dedicated server, 27,000 users.

We'll be running along fine with loads of .5-2.00, etc. Then suddenly, the loads start climbing to 50, 70, 85+. We've been trying to figure out why. We even went on a different (much more robust) server, still the same result. This only started happening after the upgrade. Restarting the server corrects the problem, but only until it happens again. It can happen at anytime - during peak or off peak. The server may run for an hour or two until this happens, or it can happen 10 minutes after a restart.

Late last night when loads were normal we benched the server hard, and it ran just fine. We just can't figure out where this load spike is coming from.

Server info below:

Servers:
Mysql: server version: 5.0.27-log
PHP: PHP 5.1.6 (cli) Zend Engine v2.1.0,
Apache Server version: Apache/2.2.3
Kernel: 2.6.19-1.2911.fc6 #1 SMP

Server:
Pentium III with 1 gig of ram
15k RPM SCSI Raid 5
1 Gig RAM

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[root@forums forums]# cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 8
model name : Pentium III (Coppermine)
stepping : 6
cpu MHz : 861.001
cache size : 256 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca
cmov pat pse36 mmx fxsr sse
bogomips : 1722.41

processor : 1
vendor_id : GenuineIntel
cpu family : 6
model : 8
model name : Pentium III (Coppermine)
stepping : 6
cpu MHz : 861.001
cache size : 256 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca
cmov pat pse36 mmx fxsr sse
bogomips : 1721.51

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my.cnf:

[root@forums forums]# cat /etc/my.cnf
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
log_error = /var/log/mysql_error.log
log-slow-queries = /var/log/mysql_slow_queries.log
safe-show-database
old_passwords
back_log = 20
skip-innodb
key_buffer_size = 256M
myisam_sort_buffer_size = 64M
join_buffer_size = 1M
read_buffer_size = 1M
sort_buffer_size = 2M
table_cache = 600
thread_cache_size = 384
wait_timeout = 35
connect_timeout = 10
tmp_table_size = 64M
max_heap_table_size = 64M
max_allowed_packet = 64M
max_connect_errors = 10
read_rnd_buffer_size = 524288
bulk_insert_buffer_size = 8M
query_cache_limit = 4M
query_cache_size = 48M
query_cache_type = 1
query_prealloc_size = 163840
query_alloc_block_size = 32768
default-storage-engine = MyISAM

[mysql.server]
user=mysql
basedir=/var/lib

[mysqld_safe]
err-log=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
open_files_limit = 8192

[mysqldump]
quick
max_allowed_packet = 16M

[myisamchk]
key_buffer = 64M
sort_buffer = 64M
read_buffer = 16M
write_buffer = 16M

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Relevant Apache:
StartServers 10
MinSpareServers 10
MaxSpareServers 15
ServerLimit 256
MaxClients 90
MaxRequestsPerChild 1000

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