Could anybody help me with CACHE problem?

Coudl enybody help me with bad caching in my web-site?

I'm user of cvs version of Drupal (i installed cvs version few weeks before 4.7. version). I can't upgrade now, because I maked many changes.

Problem: my site is running very very slow. I have about 5000 IPs per day, about 5000 nodes, 5000 registered users and about 30 registered users in one time.

My site is VERY slow. When I turn-on caching I have big problems in Internen Explorer. When I go to any page I see 2 - 3 reloading the same page with few second delay between itselfs and then page is generated but the right menu is not loaded fully. Sometime more sometime less, sometime all. But most of all the code is cutted and unfinished. However, even When I turn this crazy cache, site is slow :(

I'm very frustraded - I tried all any ideas. And now I'm very very frustrated. Could anybody help me?

Server: 2GB RAM, DB size about 120 MB.
Thank you for any clue.

The site is: http://www.moje-rodina.cz