How to put this?? err it does't always go away. Is it likely to be caused by inadequate water volume/velocity from the cisern, or poor flow rate in drain? As I live in a first floor flat, I would be suprised if it is the latter, unless the cast iron pipes have reduced bore.
Start off by making sure that the under-rim area is free from scale (and the pan too, of course). Nice strong descaler...will improve velocity of water round pan...
Also, do check that most of the water in the cistern is actually moving down the pan. Take the lid off the cistern, and look what happens when you flush. For one thing, you may have a dual-flush cistern which needs the handle holding down if you're flushing Solids - without the handle held down a smaller volume of water is used (there's a little hole in the syphon mech which admits air, breaking the syphonic action, which is covered up while the handle is down; sometimes that covering-up action breaks, and a bit of plastic glued over the 'ole converts all flushes to Big-Job-Sized flushes (at the expense of the planet)). Or you may find that the float needs adjusting to fill more of the cistern.
And the other shift-it technique which avoids handling materials you'd rather not handle is to fill a decent 2-gallon bucket from the bath, and upend it swiftly into the bog. If the problem is in the cistern rather'n the outflow, this'll give a good healthy flow which will carry everything away. If you've a blockage, though, you'd better hope that the volume of bucket is no greater than the volume of the bowl, otherwise you'll end up with bogwater sloshing up over the edges of the rim...
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