[jtroutma]

Try putting some plumbers tape on your cap threads and then tightening it. That will make sure that you have a much better seal than just pipe threads. BTW I loose some water in my res. as well, about 1/2" per week due to evaporation (need another radiator to keep the water temp below 30C to avoid that)
One thing to check is to take the cap off your res. and look inside the cap for water dropplets. That will tell you if water is evaporating or not and then depending on how much your threads dont seal, how much water you are loosing.

Just a suggestion.