I suspect that the worst possible things would be old refridgerators and heaters(radiators) with mechanical thermostats. Loads of contactbounces. I remeber having problems with our fridge and it wasn't even on the same phase.

Digital filtering like Skimask suggests would improve but never get rid of the problem completley. Your clock will always be "fast" unless you do this right.

I also remeber that the mainsfrequency was considerably lower during daytime and higher at nighttime. The electrical producers compensated for the high load at daytime by increasing frequency at night. Over a 24 hour timeperiod there should be almost no error atall. Thought i'd mention it since you didn't like drift due to temperature.

You also have the option of a temperaturecompensated oscillator, Dallas(now Maxim) makes many but this one should probably be good enough ..... http://datasheets.maxim-ic.com/en/ds...z-DS32KHZS.pdf
...... not cheap but it solves most timekeeping problems.