How to fix my ST rde 475 amplifier?

For some time the volume would attenuate for periods frm a cuple of minutes up to 10 or more minutes. Finally the surround & centre speakers ceased working. Front speakera & sub still work fine. A friend and I re-soldered a couple of joints and the speakers worked witht the test tone, howeber would nto work with say DVD. Soon after test tone would not work on the centre/rear speakers. Any thoughts?