your clutch is warped from excess heat ////sounds like broken damper springs in the clutch disc have you been dropping the clutch by any chance?
It's much better for the drive train that way.
Vibrating
mostly every dirtbike over 50 cc have a clutch so yes the xr80 do have a clutch
It's advisable to clean and lube your clutch after every ride or two - depending on the conditions. If you drive a lot of trails/sand/dusty conditions, its best to do it every ride, but if you're mostly on the street every 3-5 times would be fine. Your clutch could be worn out, but you can try to clean and lube it and see if that takes care of it. Otherwise, max-torque and comet clutches are widely available for it. With the engine off, and cooled down: Remove the Clutch Cover Blow out the clutch with compressed air (from an air compressor) Oil the clutch BEARING with a few drops of oil. MAKE SURE OIL DOES NOT GET IN THE CLUTCH Roll the mini bike around to get oil throughout the bearing surface. Install Clutch cover. Try to ride! If that doesn't work, just get a new clutch - should be about $25-30 shipped.
Yes is can if your clutch becomes on fire. make sure to replace your clutch every other day.
every year
loose washer
More than likely you have a warped clutch disk. This comes from over-heating it. Racing or ridding it at a stop light, or on a hill. If you use your clutch as a brake on a hill, (lots of people do and don't even realize they are doing it) that will do it every time.
Every time you engage the clutch, the clutch disengages from the transmission/engine. Which in term engine revs up, until you disengage the clutch.
Two every two to three weeks. Sometimes the female will lay one egg her first clutch. But two almost every clutch after.
The clutch on my Honda CG125 lasted 13500 km of heavy every day use. If you do not gear down a lot your clutch will last double that.
Trust yourself while every heart is still vibrating i think