Just a thought. I’m not a professional developer but I have had issues with hosts who are not as clued up as they probably should be! I’ve moved from Joomla to WordPress and have found it to be leaps ahead in terms of security etc.
Have you checked the security section of the support pages – loads of useful tips. And there are a couple of great security plugins too.
I’ve only experienced ‘hacks’ as a result of certain plugins which, unless you want to pay good money, is the result of getting all this stuff pretty much for free!
Good luck and don’t throw the baby out with the bath water as the saying goes!
Just had a similar problem with Chrome using XP. My WordPress kept showing banners but only in Chrome. So I did the obvious and ran a quick virus check and found an infected cookie. It’s been resolved and all is fine!
