how do i stop my toilet bubbling after flushing?

I have a downstairs cloak with a toilet and sink as well as an upstairs bathroom.The downstairs toilet worked fine up to a week ago when it started to do the following:when you flush, the pan fills with water,then drains slowly until there is hardly any water left.Large bubbles of water then erupt from the pan for quite a while after.Ihave had the drain jetted and there is no blockage.If you remove the inspection plug from the soil pipe the toilet flushes normally.the soil pipe does not have a vent to the roof but goes straight to the main drain.Any ideas would be welcome.many thanks