Selected Theme won’t stay. Goes back to default theme

Greetings,

I recently updated my blog with a 3 column, widget enabled, theme. I had been using a 2 column non-widget design which worked fine. With the 3 column design, it appears to activate and everything looks great however, after a few hours – generally over-night – the site reverts to the default theme, even though the 3 column design is still shown as active in the theme area of the program.

So far, I have tried 4 different 3 column themes (fluid and fixed width) with the same results. I have disabled all plug-ins. Not sure what else to try.

Any suggestions greatly appreciated.

Thank you.