At first I thought this was a problem with the bulb, but as I read further into your description of the problem the more it sounded like a control processor issue. The fact that the TV acts normal when the volume control is pressed, indicates that the control processor might be going bad, or at least some of the processor biasing circuitry might be going bad. The circuitry in question is somehow linked to either the bulb control circuitry.
You will have to have a qualified TV technician look at this set to determine what the problem is
I hope this helps,