You didn't say specifically what PAR36 lamps you are using. The lamp life on those varies considerably. I don't use too many PAR36's but I do like the Halozene ones.
Also worth asking is if your fixtures are open top - ie the lamp is exposed to the elements, or is there a plastic cover or some other lens on top?
Another thought would be to convert them as they burn out to a quality PAR 36 LED retrofit. I've seen many and they pack quite a punch for the watts used. Make sure what you use are damp location rated or better.
RE what steve says... yes.. the electric co can come in and put a voltage meter on the house line to track the voltage level over a couple of days on a graph. Keep in mind, however, that a 10 volt surge or fluctuation on the line voltage side, only results in a 1 volt differential on the 12 volt side.
How long are those wire runs? what is the total wattage on each of the runs? With load, those lamp readings are where they are. If you were to take one lamp off line, see what the voltage jumps up to. If you have 35-36 watt PAR's, I'll bet the reading will go up at least a volt. What this means, is that as soon as one lamp burns out, the delivered voltage to the rest of the lamps increases and so on. You might be better served to drop the taps down to 12 volts, or at least on the line with the PAR's. You could rewire a home run that feeds the 3 par lamps and drop the tap down is one other suggestion I'd offer.
GOod luck.. let me know how you make out