This volume issue has puzzled me since I bought a used Flextone III last month. For me it's a perfect little amp, as I'm just a casual electric guitar player. But, whenever I use it the volume and bass drops levels drop drastically after a minute or two of playing. The amp would kind of settle in and work, but there was still some strange behavior with levels of certain things.
Last week I noticed some other strange behavior after I found and installed the Line 6 Edit 3.04.0 software. (FWIW, this software is the key to making the most of your Flextone III.)
So, after I installed that software I start tweaking some things. The strange behavior I noticed was that I actually saw the "DRIVE" knob in the software on the computer was moving down steadily without me touching the actual amp knob or anything in the software. I wasn't playing the guitar, so it wasn't because the amp was so loud it made the knob twist on the amp itself. So I started twisting the actual knobs on the amp and found that most of them didn't track on the computer screen and match the positions they were on the actual amp. So, something was wrong.
Then today, I read this:
To summarize, the last post on that thread says to spray some contact cleaner in the volume pots on the amp and it'll fix the "Volume Issue." I started thinking about that and it made sense to me. I removed the electronics box from the amp and looked inside. I noticed that those potentiometers (DRIVE, BASS, MID etc... pots) are the open design, not sealed like many of them are today. I'm an electronics tinkerer and I happened to have some pot cleaner spay in my shop. So, I sprayed inside each pot and twisted them each knob a few times to make sure the cleaner got into the pot and distributed properly. I let the cleaner do it's job for about an hour and twisted the knobs again a few more times.
Before I installed the box back into the amp I plugged it in and connected the midi cable to my computer to see if that fixed the knobs from moving on their own and track properly. Success!
Now, when I turn each knob they track perfectly in the software on my computer! They don't move on their own anymore too. So, I put the amp back together and have been testing it with my guitar plugged in tonight and it's like a whole new amp. The volume doesn't drop a minute or two after I open a new preset. The onscreen knob positions are perfectly in line with what the position on the actual amp are now.
I'm convinced this is the "fix" for the Flextone III volume issue. I'd be happy to provide more detail if anyone is interested. Maybe someone else will try this and report back their findings. If anything changes I'll report back.