I have got a 2000 model 250 which got cracks on both sides of the frame, my friend has the same model bike with exacly the same weld problems, easily tig welded just steam clean the tank first!!!!
I always put it down to the bike doing the odd cartwheel, but who knows?
p.s. I also had a problem with the original swingarm that broke along the weld, fitted the 02 type, much better.