For the last few weeks, my Harman Kardon HK3400 has been driving me up a wall. Both the left and right channels would cut out constantly; often in very short bursts. It got to the point where the music was unlistenable because it just got too distracting.
I did some research and experimentation, and it looked like I had a classic case of dirty potentiometers. This is common in old audio equipment because the contacts of the potentiometers will oxidize over time and cause a poor connection.
Luckily there's a pretty simple fix. The first step is to take care of all the dust in the case. Simply open the amp up and blow out all the dust with some compressed air. I used an air compressor, but canned air will do. Make sure to clean as much dust out as possible, especially near all the potentiometers. At this point if the amp wasn't producing any sort of scratchy sound when moving the knobs around, you should be good to go. If not, you should use a contact cleaner on the potentiometers directly. One product most people recommend is DeOxit. Here's a great guide on how to use it from user Arkay on AudioKarma.org http://www.audiokarma.org/forums/showthread.php?t=207005
In my case, I was done after a simple dusting :)
These old amplifiers are incredibly well made, and will stand the test of time if you treat them well. A cleaning every few years will make these amps last a lifetime. Mine's been going strong for over 30 years!