Gears braking - two times in a row

[matt88]

Hi, I have recently bought a gasgas 300 ec 2011, and after 2 hours of riding - third and fourth gears jammed and were trashed. I thought - stuff happens. After rebuilding and riding 2 more hours - same thing happens - the same two gears 3 and 4th jam together and brake. This happened both times when i was downshifting - first time i wasn't using the clutch, the second time - i did pull in the clutch. I had many bikes in past and this has never happened while downshifting in the past. What could be the problem guys? My mechanic is an experienced KTM mechanic, he couldn't find the cause for the gears to jam and brake - doublechecked with another great mechanic - no ideas. I was using BelRay gearsaver 80w one time and another time it was Maxima MTL 80w. Any idea guys what could be causing the problem repeatedly?