I just bought a sx-950. It worked great for about 3 weeks. Then I noticed after it had been running for about a half hour it lost power to two speakers. I have never opened it up. Dirty? or something else. By the way i got it for 30 bucks on my local craigslist. Help.
I would bet that some component values have drifted over the years and it needs to be re-biased. The DC offset should be checked, too.
When an amp gets warm, some parts do crazy things and this one may be in that category. As mentioned, it's possible that there's at least one bad solder joint. I would also clean the controls and switches. At that age, it's due. While you're at it, remove the cover and blow out any dust. Use a cheap paint brush to get into the crevices and tight spots.