I have had a problem re-appear after appearing to be "fixed". Appearances CAN be deceiving!!!
I bought a 2nd hand 300, with the intention of transplanting the electrics into a strat clone.
I had already replaced the jackplate on the 300, cos it appeared the 1/4" out didn't work, and the first two piezos would only give any "audible" signal after having being connected to the Pod HD500 for "X" minutes. I say "audible" as I could tune the guitar straight away using the Pod's tuner, but on engaging a patch, only string 3-6 would be "audible". This glitch would correct itself, sometimes quickly, sometimes after being connected for quite some minutes and the guitar was fully playable across all 6 strings.
This in itself tells me, the problem wasn't a piezo problem, Even so I transplanted the 300's guts into a strat clone and in doing so upgraded the piezos to graphtechs. It was always my intention to do this, and I also did it with my 600 which was my "workhorse".
After a bit of time and effort, re-doing, and re-checking my soldering, and the ground contacts, the 300 transplant was working fine, across all strings, volumes lowered on all strings to around 70% to match the output of the 600. Beautiful!
Recently tho, the two intermittently "dead" strings problem has re-appeared. I can only figure it must be another contact (I think unlikely), or a PCB issue. The two strings come back "to life" at the same time. If it was prior (and still is) a PCB issue, why would it go away, then re-appear.
Also I haven't checked it out audibly (ie with my ears), but I always plug the 1/4" out into the guitar input of a "Live 3" vocal effects pedal, and I'm pretty sure the light on the unit doesn't "go on" (showing input signal) when I play the 300 xplant, but it does when I play the 600 xplant. I need to check this too, as it could even be related.
Any "idea" input would be appreciated.