Loewe TV with pin cushion distortion

Question

After approx 20 minutes once TV is switched on, the screen distorts with a pin cushion effect - the sides of the picture curve inwards. If the set is left on, the apex of the curves become staggered and the picture that remains, reflects this distortion across the screen. Can you please advise the probable causes and possible cures. I'm an end user with an electronics background presently unable to locate a repair manual.