Subsite siteurl option getting reset to top level site

Hi,

This isn't a question for WPMUDev premium support, but just reaching out to the multi-site community for ideas on this one.

I have a multi-site that is acting strangely. The mapped domains are working but subdirectory sites break after a short while.

After investigating, I have found the 'siteurl' option had somehow go set to the top level site, so instread of siturl= http://toplevelsite.com/subsite it was reading http://toplevelsite

So I reset it, to the the subsite, and hey how, all workd well for an hour or so then it gets overwritten.

No it is a bit tricky for me to debug this by turning all plugins off as there are live sites on this multi-site and recreating in test will be a big pain.

So then I thing perhaps wp-cron if fireing something so using crontrol I have fired off the various wp-cron jobs, still not able to create the situation. But an hour or so later and the siteurl is wrong again.

So maybe its the theme? Maybe it is some user interaction with the theme. I'm off to create a test subsite with the default theme, and see ... but any one with any ideas please chip in.

Alan