This almost always caused by the furnace tripping on high temperature limit, or some other safety limit. When the temperature limit is exceeded, the furnace shuts off the burners and the 24VAC to thermostat. When the stat loses power, the clock stops. The furnace blower remains on, so the homeowner may never notice a problem, and when the temperature is back within limits, the burners come back on, and the power to the stat is restored. The clock will be slow by however many minutes the power was off to thermostat. Correction: Find the cause of the high temperatures - dirty filter, etc.