[old_tv_nut]

OK, you have snow and no intermittent with the back off.

I'm stumped, but would try putting the back on again.

Having snow is normal, but if it goes away to a blank picture again, it means that the signal path is broken somewhere in the tuner or IF or the connection between them, or the AGC voltage has gone wacky. How putting the back on could cause this, i have no idea, but that's the only indication you have so far. Could it be a heat problem opening up a connection? Don't just tap the tubes, grab a cloth or something to keep from burning your fingers, and wiggle the tubes.

Edit: Also, what happens if you wiggle the channel selector?

Normal operation with no signal should always give you snow.