The transmission on my YZ426 took a dump when it was 5 months old. The problem started with it jumping in and out of 4th gear very rapidly at WOT. It started occasionally then progressed worse. It was finnally diagnosted on a Dyno where we could actually see the chain whipping around because it never actually pop all the way out of gear. After being tore down, 4th and 3rd gear were thrashed and apon further inspection, using the service manual for reference, we discovered that a shim was installed in the wrong position. After a long wait for for replacment 4th gear the trans was assembled with new gears according to the service manual and since then has been trouble free. Any comments?