I presume that the bulb in your enlarger is a conventional incandescent bulb.

The light output from incandescent bulbs varies SIGNIFICANTLY with voltage and is likely to be the cause of your exposure problem. A 5% reduction in voltage (which is within the range of variation that a utility may apply during normal operations) results in a nearly 20% reduction in light output. A 10% reduction in voltage (which is unusual, but possible) can reduce light output by up to 70%! And as the light output varies, the color temperature of the lamp also changes - the output becomes noticeably more yellow as voltage drops.

The impact on timer accuracy not nearly as profound. As you say, if your time looks at zero crossings, then the timing rate is controlled by frequency rather than voltage magnitude, and frequency is unlikely to change dramatically (unless you are in India where frequency is notoriously variable). Voltage variations can affect the timing accuracy of motor-based timers, but I would not expect that magnitude of error to be nearly as significant as the magnitude of lamp output for a given variation in voltage.