2008 Lincoln MKZ My tire pressure is constantly low

I have a 2008 Lincoln MKZ with 50k miles on it. I have had it for 19 months. Ever since I purchased this car, I have had a problem with low tire pressure. At first the Low Tire Pressure light would come on every week or two. I also had a couple plugs put in due to nails. It had Michelin tires on it. Over this past summer, I had to start adding air once or twice a week. I took it to the dealer many times only for them to tell me the tread was ok and they could not find a hole where the air could be escaping. I finally got tired and bought 4 new tires this past weekend on Saturday 10/23). Everything was fine till this morning (Monday 10/25), the Low Tire Pressure light came on again! I called Merchants Tire (who installed the new tires) and they said it could be because the temperatures dipped low last night and that it should resolve itself when it warms up today. Needless to say, it did not. Could you pleas tell me what could possibly be wrong and what my next steps should be? Should I take this up with the dealership? Could there be a larger problem that is causing this to happen that might be a dealership/warranty issue? Thanks for your help!
Gina
Gmoni7@aol. Com