We have had a rash that sounds like you describe - light and all over. It was also on and off, it went when I used Sudocrem or a soothing cream on her at night only to reappear the next morning after a couple of changes. It was made worse by PUL AIO's and covers and started when the weather got hot and humid. Turns out it was thrush - I used Daktozin on her, just at night, and her little bum is lovely and smooth again Just a thought....

If you try this though, do use a microfleece liner - it is awfully sticky and would clog up nappies awfully without it.

HTH and hope you get rid of the rash soon