I have a WP blog that loses it's theme over and over - ideas?

[Bill]

So, I've got this old blog that has a strange problem.

I put a theme on it, and the blog runs for a couple of days, then it loses it's theme, and reverts to a default theme.

I took out the default theme - and the blog loses it's theme, and crashes to a white page.

It does this no matter what theme I use, so far. It didn't use to do this ever, for years it ran a theme reliably.

Any ideas of what is going wrong? googling 'wordpress loses it's theme' and searches in the wordpress forum have turned up nothin so far.