My downstairs toilet has suddenly started draining slowly (it's been fine since we moved in 3 years ago). It now take about half a minute to empty the bowl, and doesn't cope well with solid waste/toilet paper. At first I thought it was a blockage, as it seems to clear it after a good plunging. However, I come back to it some time later and it's back to draining slowly? This has been going on for weeks. It's the fact that it's so intermittent that's confusing me. Even without use it will go back to draining slowly...? Tried plunging (various types), drain snake (useless), drain cleaner (no effect). Any ideas?