editing problem

I started to talk about this in another post but thought it should be under its own topic. I am having problems editing a theme that came packaged with WP installed by Dream Host. I am getting a white screen now and then when I try to post a message or navigate my site from the control panel (the site seems to be working fine on its own). Also, when I try to edit the post, the changes are not saved – sometimes I get a white page and sometimes not. I am also unable to cut and paste. I tried rebooting and deactivating my plugins. I also looked around the archives and found things about reinstalling WP or themes, or stuff that looks like it has to do with code. I am using a WP install from my host with a theme that was included, but I did try to install some themes that don’t work. Do I need to uninstall them (how)? Do I need to get my host involved? Am I over my head already and need to get someone that knows what they are doing? I experimented a bit with WP.com to see if I could get it and didn’t have any problems.