Yo Jim,
Look into the following possibilities:
1. What kind of wire was used in your home...copper or aluminum? If it was aluminum you might be getting some arcing in the receptacle that the B&K is plugged into. It will read fine on a voltmeter, but under load the voltage will drop.
2. In either case, check your connections; first find out which breaker the outlet your B&K is connected to. Turn everything on in that part of the house: lights, fans, plug a table lamp into receptacles with the light on, and go flip the breaker off. You should find about 4-5 loads (lights or receptacles) that go off. Take the cover plates off the outlets and switches and pull the devices out of their wallbox and check the wires for evidence of carbon or heat. Tighten all screws and wirenuts. Install a brand new hospital grade (Hubbell or Leviton) receptacle for the B&K outlet. Reinstall devices and cover plates. Check to see if the problem is gone. If not, goto step 3.
3. Check the connection at the breaker. You will need to remove the cover from the panel, but before you do, turn off all the breakers and the main (if you have one)...there are 4 or 6 screws that hold it on. Once it's off, you'll need to check 2 wires: a black connected to the breaker and a white connected to the neutral bus (finding the right neutral might be difficult as there are usually 2-4 neutrals coming in to each knock-out so check them all).
4. If every thing checks out ok, then you might want to have a new dedicated circuit installed for the stereo...it's an option that is usually only feasible in
houses with an attic or crawlspace. It might be cheaper than a large UPS.