it's simple --- upgrade.
cpanel needs atleast 512 memory to run "decent" (also depends on cpu & doesn't include the services that are running.).
so you only have about 1GB left for your website which doesn't seem to be enough.
however.... you could try optimizing mysql (in ssh: wget mysqltuner.pl -- then chmod +X mysqltuner.pl -- then perl mysqltuner.pl -- and follow what it says)
what php are you running, and are you using dso, suphp, etc.
apache ver?