Make sure your anode (the clip under the suction cup) is securely connected and the cup is making good contact (You might want to discharge the tube before you do this)
If those are good look for arcing around the flyback transformer
If the former is bad but not completely separated, you often get a sizzling, popping sound. A rusty anode button or hook will often do this, and can be cleaned - once discharged, of course!
For the latter, or any corona, turn the lights off and look in the dark. If you have to do that to see it, it's probably not the cause of your trouble, but it's worth checking.