My cache partitions are filling up, what do i do?

I am running Squid 2.2Stable4 on RH 6.0 with 2.2.12 kernel, 128MB RAM
and 2 4GB SCSI disks. I assigned 2 250MB partitions for squid,
/squid_cache1 and /squid_cache2 and every now and again one of the
partions will fill to 100% and squid will die. These are the relevant
parts of my squid.conf:

cache_mem 64 MB

cache_swap_low 93
cache_swap_high 96
                   
cache_dir /squid_cache1 240 16 256
cache_dir /squid_cache2 240 16 256

The partitions are 250MB each but I set the cache_dirs to 240MB so that
there was no chance of them filling up, but they still are...what am I
missing? thanks,

-- 
Fernando Medina Jr.
<fmedina1@dla.co.cr>
ICQ 102329
San Jose, Costa Rica