What is the problem with my Marshall JCM-900?

Take it to a shop.

I was going to recommend changing the tubes but since you’ve tried that it probably means there is a wiring problem or something complex like that. Take it to a shop and see what they say.

Taking it to a shop would be your best bet.

I’ve always used solid state, so I’m not for sure, but don’t some tube amps need time to warm up? I can’t remember where I heard that, so I could be wrong.

Yeah, it warms up, but after a few minutes of playing fine, it fades out and in.

Yeah, I’d say it almost has to be the wiring. How old is it? Does it still have a warranty?

Your problem can be addressed two ways:
1. The tubes are biased improperly. Take it to a shop.
2. One or more capacitors have corroded or dried out. Take it to a shop.

There are online tutorials about self-biasing, but I don’t recommend them.

Capacitors can kill you. Do not touch them.

If you bought your amp used, there is a chance that it sat for a long time before it came into your hands. Tube amps need to to be played through often for optimal performance.

If I had to choose, I would go with bad caps, and unbiased tubes exacerbating the situation. Take it to a shop. While you’re there, make friends with the tech, and learn some stuff. Good luck.

Besides what’s been mentioned already, the problem could even be as simple as bad speaker cable or connections. So you might want to check that out before spending money on repairs (repairmen aren’t always honest about simple problems).

I’ve tried the cables with no luck.

I used to work on electronics as a hobby back in the tube age. I agree with @Knotmyday. Don’t mess with capacitors, even with the power off, unless you know exactly what you’re doing. The stored up charge can kill you

I have 2 jcm 900s and both after warming up started to cut out and fade out. On the first one it turned out to be poor contacts on the effects loop. take a regular guitar cable and run it in and out of the loop. If it works the contacts need cleaned. wd 40 sprayed in the holes took care of it. The other was the channel switching jack on the front, same thing needed sprayed. Set both channels to the same volume leave the overdrive channel button out. The fading should stop. Just for the record I removed the el34s and replaced them with JJ kt77s OMG what an improvement. the bias needed turned up a pinch to about the 2 o’clock pos for maximum tone. That requires removing the chasse and you will see a black pot on the back board