Now that the bikes are getting old, most have to be gone through just as preventative maintenance.
There have been several guys discussing third gear dogs wearing and popping to neutral, (it may be third is just used more than the others). It seems the key in the flywheel pops up a few times a year, and the carb body, slide & needles have usually seen better days. Most guys who have been around a while will go through a new purchase just to be sure all is well. A gasket kit and consumeables will run about $100.00 and can be marked off as "Insurance", and getting everything up to snuff will give you a good baseline to work off of.
Tuck\o/