Husqvarna experts help please with WR250

Please help, i am out of ideas. I have a 2004 anniversary edition Husqvarna WR250 2-stroke. It has new pistons etc at a cost of R7k. Every time i ride i have to replace the spark plug. Then it goes fine until I stop riding for an hour and the engine cools. I then start it and it blows a plug in under a minute. It always blows plugs after the ride when it has cooled again. Replace the plug and then it's fine again until it has cooled. I have changed more than 10 plugs on this bike already. At R40 each ride. The plugs seem black and oily and wet. I have had it back at the dealer and had experts check jetting and everything. Next ride it happens again. Please advise what this could be other than bad karma for changing from an orange bike