Can coect to Internet, but not Google properties

I have this recurring issue where the my machine will be stop being able to coect to GMail, Google Search, Google Fonts, Analytics, etc. but will happily serve any other URL at full speed. I can watch in Chrome's Network panel as it takes two minutes to download one 100kb xhr for Gmail, while in another tab I can browse pretty much anything I want (provided it doesn't use Google Analytics, Fonts, or Adsense).

It really seems like the restriction is to Google properties only.

I've tried using Google's DNS with no luck. Rebooting the router has no effect. All the URLs are SSL, so it seems unlikely that there is any MitM action going on (even though the ISP is BT who would not be above such things). I'm at a loss.

Any ideas as to where to even start trying to debug this?

Edit: This might be limited to Google Chrome (Mac), as I can occasionally get into Gmail using Safari when it's happening.

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