#1
have a crate half stack, my problem is when i play the sound will fade in and out? and also i cant switch it from b channel to a it only stays in b?
#2
is it tube or solid state?

sounds like a hardware problem, i would recommend taking it to an amp tech
#4
I've heard some bad stuff about the Crate amps. Something about the electronics really going to crap, especially when you're playing at a high volume. Your footswitch may be busted or something internal in the head is fried.

Take it to your local repair shop and have them look it over.