You can "roll your own" by connecting input to center terminal of 100k pot throught 1uF cap. Connect outside terminals to pins 2&3 of LM386 chip, then connect pins 1&8 together through 10uF cap. Pin 6 goes to base of high gain NPN transistor, emitter to +V, collector to ground through...
Sounds like intermittent capacitor(s) or bad solder joint. It may be a ROM problem or IC loose in socket. Are all the peripheral boards in tightly or are there any loose wires / connections. Check for RF interference. Is the power plug in tightly ? It may be a bad peripheral, unplug them and try...
