I've had this issue for about 4 months. When I first access my website www.757sc.com on a browser, upon the first click on my homepage, a tab opens that is usually an ad or the ramsom type of site directing you to call the number to unlock. I initially thought it was my computer or browser, but this is an issue in Chrome, Mozilla, and IE. I accessed it via my phone (safari) same issue. I tried my home computer and laptop, same issue. So it appears to not be a device issue. I called Shopify support and the said they can not replicate. The very concerning this was somebody I buy products from accessed my site, and said they had the same issue. I am still doing strong business, but am wondering if I am losing business from this? Or if it is happening to customers. Any ideas what in the world I can do to stop or fix this? I have so much time and money in this site that I don't want to lose all that time and effort. Thanks for your help.
Wow, 4 months is a very long time. I can see the issue you're talking about but it's not on all pages.
Here's what I can establish from a quick peek:
Pick an effected page in Admin and view HTML in the RTE (rich text editor). What do you see?