Volume of a certain strings is changing (piezzo)
 

On a Yamaha piezzo-equipped guitar, I'm getting problems with sudden weaker output of 5th and 6th string. The interesting issue is that, if I use my fingers and mess a little with a spot where the string touches the saddle, I'm getting the right (stronger) output, but then it goes away after a while (and comes back again when I move the string, or knock on the saddle a bit). How to solve this?

It is possible that the saddle slot is not perfectly flat, or the saddle bottom is not perfectly flat (maybe due to wood movement), that the saddle has developed a fracture, or even the transducer has gone bad. A few slips of paper between the saddle and the transducer under those two strings might be a cheap fix. A new saddle is the next DIY option. Then to a repair shop for more complicated work.

I have put a flat thin layer of super glue under the saddle's high e area once. It worked and gave more contact between the saddle and the piezo thus increasing the volume of the e string a bit more. Ideally, though, I'd spend the time and effort and have a good luthier look at the guitar (if it's the kind of guitar worth spending $100+ on, that is).