Seems to be up and down like a yo-yo. :(

Did you flick the stability switch the wrong way?

I think it likes to change itself :/

I've put it back on prefork mpm because worker mpm seems to go nuts and I can't find a good balance of performance and resource usage.

We're still on FastCGI PHP for now, even though that doesn't appear to actually do what I thought it did, but it does save us lots of memory which is good.