I'm getting the same thing - I thought it was just our internet speed, but it's interesting to find out others are having the same problem. Very frustrating when you're trying to update pages and receive some random message that talks about Gurus and Varnish.
This is a recent problem which I think is due to misbehaving web robots trying to index pages of the system that don't exist, and causing excessive database load.
I've made a change which I think will help (it catches these invalid requests and redirects the robot somewhere more sensible).
I hope that this should have a positive effect on the performance, and thus also get rid of the 503 errors.
If the problem persists please do post on here again about it and I will investigate further.
Joe - Voice Admin