MySQL Server crashing more and more often as site grows

[findingpeace]

On pretty much a biweekly basis now, our vB4 site is crashing and I get a million "vBulletin Database Error!" emails. Most of the time, the database gets corrupted and needs to be repaired. I'm providing my server & forum stats below, in hopes that someone can recommend some tweaks for me to stop all of this downtime?


FORUM INFO
vBulletin Version: 4.2.2 PL1
Total Size: Threads 17,291 Posts 247,138 Members 7,945
Posts per day: 800
Google Analytics Daily Visitors: 5,000
Google Analytics Daily Page Views: 30,000
Resource Intensive Mods: DBTech vBNotifications (15 second refresh)


SERVER INFO
KnownHost VPS SSD-3
3584MB Guaranteed RAM
50GB RAID10 SSD Storage
3x Priority 24 CPU Cores
4500GB Premium Bandwidth

MySQL conf file:

thread_cache_size=4
table_cache=512
local-infile=0
innodb_file_per_table=1
query_cache_limit=1M
max_connections=500
key_buffer_size=16M
join_buffer_size=1M
query_cache_type=1
query_cache_size=32M
max_tmp_tables=1
default-storage-engine=MyISAM
open_files_limit=4974
max_allowed_packet=268435456
innodb_use_native_aio=0
innodb=on
innodb_buffer_pool_size=134217728

People automatically say "raise the refresh rate of vBNotifications!" but this really should not be a problem. Our site, even at peak hours, runs at only 1-4 CPU load average (10-20% total), and 1GB memory free.

The site just randomly craps out and crashes. Nothing to do with unusually high load.

Any thoughts?
Thanks!