Did the initial install checklist that shows up when you first go to install Concrete5 show everything ok? If there were any things on there without a check you may have to mess with your php options file for your local server, or change some directory permissions.

I'm using MAMP (for a mac) and it turned out well. Maybe you could try a different LAMP auto install program and see if that works.

Install checklist was all OK. Baffling thing is that it works 80-90% of the time and when I refresh the page after the error it works.

Ok, I looked up GetRow() in the concrete5.org sitewide search and it seems that function has something to do with MySQL. I only saw error messages that had to do with switching a database from a windows computer to a linux computer, but maybe that'll give you a (slight) idea where to look for your problem. If I were you I would delete everything and start from scratch, with only the core files (no extra blocks or anything), and then keep track of what you do until (if) the error shows up again. You mentioned lots of pages, so either you have installed the default content, or you have your own stuff in there. If you switched from a windows computer to a *nix computer (or vice versa, I'm assuming), and you imported a database from another install, you'll have troubles with the tables being in the wrong case. There are some other threads in this forum that will fix those issues.

Also, I would definitely delete the cache and disable it for the dev environment. Cache is in sitewide settings, I think.

I am still learning about MySQL and PHP but I hope that helped! :-)