Table locking generally

[Zakai Kinan]

I have a problem that I can't understand readily.  I have a database that has a couple of
tables that lock for a recognizable period of time.  The reason I know is because during
the lock the application stops responding totally.  The storage engine is MyIsam.  I have
reread everything about locks in both InnoDB and MyIsam.  I know that I can change to
InnoDB and that would alleviate the problem, but I am curious as to what the problem is. 
This is a new problem for the application has been running for 4 years.  The server is a
64bit server with 16GB of memory, raid 10, and a quad core CPU.  The size of the database
is 600MB.  The lock occurred while two users were accessing the same table.

Thanks for any insight,

/ZK