First, a speed wobble is not that uncommon and can be corrected once everything is in proper working order.
The lower section of your fork that the axel and caliper attach to should be tight on the fork leg. Please make sure and correct this issue before continuing to ride. A failure of this part could lead to serious injury.
Go to the Yamaha web site and do a parts look up on your bike. The online feature allows you to look at a parts break down of every component. You can easily magnify each view so as to see every part in the assembly. Then identify what is actually loose.
Remove the fork leg from the bike and inspect it closely. I believe that there are set screws that hold "the end cap" you are discribing in place. Most riders never have to remove this part and most suspension shops would only remove it in extreme cases. If the shiny fork leg twists, wiggles, or slides in this lower assembly, repair it or send it to a reputable suspension shop. Solving this problem may not stop the high speed wobble but will allow you to diagnose and properly correct the highspeed wobble. Without repairing the lower assembly, you will likely chase your tail trying to fix the symptom and possibly get seriously injured in the process.