I realize that I had a discussion with IG support some time ago when I had a similar issue. Basically their logs were saying that I was hitting the application limit of 60 requests per second even though I was nowhere near that rate.

I suggest you contact IG support just to see if you are running into that same issue. Provide them with the api key you are using, date, time and time zone when you hit the limit.
If it helps, the guy who helped was named Matt Bailey