crashes every few hours, i think something to do with innodb

Hello,
I have been facing this problem since past week. Server would randomly stopped working even though it doesn't now have high load average. Couldn't find anything in logs, I think it is because of the mysql crashes. There are about 4 sties whos databases are not working anymore, (even though they exist). They all went to "internal server error" all of a sudden, after restarting mysql, some of them came back and some shows no sign of database connection in wordpress.

Here is my my.cnf
#[mysqld]
#innodb_file_per_table=1
[mysqld]
max_allowed_packet=64M
log_slow_queries="/var/log/mysql-slow.log"
innodb_file_per_table=1
open_files_limit=50000
query_cache_size=16777216
long_query_time=1
thread_cache_size=2
default-storage-engine=MyISAM
[mysqld_safe]
log-error="/var/log/mysqld.log"


Here's are the last lines of mysql logs.
root@server [~]# tail /var/log/mysqld.log
140301 7:29:21 [ERROR] Cannot find or open table xeramein_phpfox/phpfox_custom_field from
the internal data dictionary of InnoDB though the .frm file for the
table exists. Maybe you have deleted and recreated InnoDB data
files but have forgotten to delete the corresponding .frm files
of InnoDB tables, or you have moved .frm files to another database?
or, the table contains indexes that this version of the engine
doesn't support.
See MySQL :: MySQL 5.5 Reference Manual :: 14.3.19 InnoDB Troubleshooting
how you can resolve the problem.


Any help would be highly appreciated as I am loosing a lot of clients because of this.

 

Also, the /tmp dir is getting piled on very very reapidly. In 10 minutes it reaches to 100% which is about 4GB. It usually used to take like 10-15 days earlier.