Sounds like a bad cap problem on the Composite, S-Video, Coax board. I would check that board for bad bulging leaking capacitors. The board is probably located at the rear inputs, the coax, composite and S-Video ports being physically attached to it, with the front inputs connected to it by wire. It could be that there are 2 separate boards for the Component and VGA inputs and the Composite/SVideo/Coax or it could be just two separate paths on the same board.
Panasonic AX-200u / Optoma Graywolf 92" / Draper Luma 92"
Samsung 51F8500 Plasma / Panasonic P50ST60 Plasma / Sony 50W700B LCD / Vizio VP322 Plasma
Sony S790 BD / Panasonic BDT-220 / Roku 2 XS & Roku 2 (Ver. 3)
Panasonic S97 / Panasonic RP-91
Aiwa MX100 Multi-region VCR / JVC S7600 S-VHS