Hitatchi DVD/VIDEO CombiDV-PF4E Interferance.

My father has one of these units and it is showing intermittent ,interference/tracking out lines.It is set for Auto tracking but every couple of playbacks (from video) the tracking lines come up the screen.Trying to adjust Tracking manually doesn't have much effect.I have tried scart connections ariel e.t.c they are fine.Incidentally it mainly occurs on playback of T.V programmes (not feature Films) is this of any relevance?Please suggest solutions!!! P.s i have recently installed a Philips set top box.Would this give any amplification to interferance.Many thanks J.H.