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