TV shuts down when receiving SCART signal?

My Toshiba 32ZD26P CRT has started behaving very strangely. I have a cable TV box plugged into SCART 1 and a DVD player/console on the component input. The other day when watching cable the TV the on-screen source display appeared (like it had just received a new signal), then the tube shut off. I thought it had gone into standby, but the TV failed to respond to the remote control.

I switched the TV off, waited a minute, then powered up and all seemed fine again. However, when trying to switch sources to the DVD player, the TV shut down again. This puzzling behaviour has worsened over the last few days. Now the TV can happily run for a couple of hours using the cable box, suddenly shut-down, take several attempts to come back to life, but then keep on shutting down again after 7 minutes of viewing! Once it gets into this cycle I'm done for the evening.

The problem is definitely something to do with switching to the external inputs. When the TV is misbehaving I can switch it on without any external connections - but as soon as I plug in the SCART lead and turn on the cable box (and the TV gets the switching signal), hey presto - shut-down time again! If I try to switch sources using the remote control, instant shut-down.

Help!