Noblues: I suspect PS - I had one machine that did that intermittently until I got a new PS.
Otherwise - is there a pattern? Same time of day or something? This can happen if your neigbour turns on his/her 2000 watt Soopa Doopa vaccuum cleaner or heater or whatever (or something powerhungry in your own house coming on or off). A better power supply might well sort that out too.