Theme not working properly - CSS doesn't seem to load always

I have installed the Spirit upfront theme to test out before I recommend to 2 clients. One thing I have noticed and had feedback about is that the theme doesn't load all the time. Eg the CSS seems to not load intermittently or it only loads part of the way. It doesn't happen all the time but it happens very frequently. I cannot proceed to design the site unless this is solved as it looks terrible. Happy to provide screen shots but it is reproduced on Chrome and Safari simply by clicking different menus a few times.

I have tried to narrow it down by uninstalling the plugins but what I have found it is the upfront themes that consistently cause the issues.

When i used twenty eleven (or unto 14) it was fine... I have tried the other upfront themes and it does the same thing.

Could you please help

Thanks

Ian