Hey! Thanks for your response.
Last night I backed up my SQL database, uninstalled & reinstalled wordpress, reinstalled the theme and restored the SQL … not sure what in all that fixed it, but it seems to be working (for now, at least… touch wood!)
I was using MailPoet – I found a line of script at the top of all the PHP files in my plugin directory – went through and removed all of those and it seems to have fixed the issue.