Forget Toyota Dealership.
They are enthusiasts just like us..... after cleaning Cat Con, TB service and fuel filter replacement their problem resolved after spark plugs replacement.....
Even i encountered same problem with my previous Honda City around 30k... some how managed to change them under warranty....
My collegues and pool cars Xlis under my control used to have normal copper plugs as we all know. They worn out at around 15k kms and replaced.... these conventional type plugs used to last till 30k easily before....