VG8235 video problem

I have a Philips VG8235/02 (like the /00 but with a german keyboard) with an unstable video output: the image continuously vibrates in the left-right direction a couple of times per second, just enough to be annoying. Otherwise, color is fine and position on the vertical axis is stable. The strange thing is that this problem occurs both when I use the antenna output and the SCART output connected to the TV's AV input. Any ideas what might be the cause?