I have a Gibson Les Paul Vintage Mahogany running through a Line 6 Spider Valve 212. As of recently, I've noticed that when I'm playing and flicking the pickup switch, occasionally the sound of the current channel would either fade out and barely be audible OR if there was any present distortion on like a metal channel, it would almost entirely disappear giving me like a Bluesy kind of overdrive for a couple of seconds until the distortion kicked in.

I don't know what the source of the problem is and I don't want to go change every possible thing for one problem. I don't know whether it's my amp's tubes and I should get them exchanged (I bought the amp used and had it for over a year, but I always played like 10%-25% volume), so I don't know if the tubes died that quickly.

Or is it my guitar's pickups? I seriously don't see what's wrong with the guitar and I only had it for a year.

Or maybe on the off chance it's the guitar cable?
get some contact cleaners and clean the connection points
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try another guitar through your amp. I'm guessing its your pickup selector because it happens when you change your pickups..
I Survived The "Silent Deftone Cocksucking Forum"!-August 15th, 2006
How do I clean the connection points? Of the guitar? I don't see how I can do that without taking it apart.

To be more specific, if I'm on a clean channel and I then switch pickups, the new pickup is then really quiet and dull sounding and then after flicking back and forth for a bit or just waiting for a minute, it fixes itself. When I'm on a distorted channel, I flick to a different pickup and then I get a dull, not as distorted sound, like a blues channel or something.

I have a guitar repair guy in mind but I don't know if that's the problem exactly or how much it will run me.
So what's the method to fix this? Is this like a self 5-minute thing I can do myself or do I take it to a guy and get it fixed?
FIRST try another guitar through your amp (if possible). Change the pickups and see if you still have the same problem you had with your Les Paul. If you are not having the problem, its most likely your 3-way pickup selector switch. You can fix this problem yourself if you know how to solder. You will need to buy a new 3 way switch, unsolder the original switch, and solder the new one in. There should be schematics and wiring diagrams on the internet to help you out. You can also take it to a guitar tech and he will do it for a charge. I don't suggest doing it yourself if you have no soldering experience but it isn't too hard (and it does suck to pay for someones labor).

Good luck. Hopefully you can quickly resolve this and get your ge-tar working normally.
I Survived The "Silent Deftone Cocksucking Forum"!-August 15th, 2006