Performance .. Prestashop kills my VPS every time I admin

[Mohamed Esmat Farag]

Hello!
I am not sure if this is me only who is facing such a strange issue.

The VPS is: 694 MB RAM
Plesk Parallel 9.2.2
Rapid SSL, all content from inside the Presta folder goes https.
We have all sites static
We use drupal only for a newsletter subscription form.
5 domains, 3 of them are complete sites.

Host Server:
-------------
centos5
8 GB
Xen Version: 3.2.2
CPU: 8 x Intel® Xeon® CPU E5310 @ 1.60GHz
Memory: 1,666 MB Free / 12,286 MB


Every time I edit some categories, or products, and after average: 15 minutes: page doesn't go through
and all sites don't show up if visited. One time, few minutes after I submitted a ticket to the hosting company, the server came back live again as normal.
Two times, the VPS doesn't come back until I reboot it through the host's VPS control panel.

Now, do you think Prestashop is not usable on this VPS?
Why that happened, while osCommerce never made such a problem, but without being worked with behind SSL.

Should I do something before I give up with Presta?

[Mohamed Esmat Farag]

i mean: "working" not "sitting" and waiting for the sever to hang!
"while" working, after 5-15 minutes of sign in as admin, the server suddenly stops.

hope i explained it better?