JVC Widescreen N64 problem

Allo All
I've had a JVC Av-28GT1 for a few months and am having trouble connecting my N64 to it. The sound is fine and the picture is OK but bounces at about 1Hz. Have tried the phono I/ps as well as all the scarts, switching it into various widescreen modes and the PAL/NTSC variations in the menu but to no avail. Wondered if anyone else out there has had the same problem or any ideas on what might be wrong. The N64 works on my portable which has phono I/Ps and worked n my old standard Mitsubshi TV via scart. I don't have an RF modulator for the N64 though.

cheers in advance

Dave