2005 Scion TC TPS System and Tire Pressure

Tires and Wheels problem
2005 Scion tC 4 cyl Automatic 106000 miles

Hi, I have a 2005 Toyota Scion TC and the last few times I have had my car serviced with brakes and tires, it was at Firestone. I also had new back brakes put on through the local high school auto tech department, as well. Ever since I left a firestone location, my tire indicator low pressure light keeps coming on and staying on. It looks like (!). I took my car back into firestone and they told me that nothing was wrong with my tire pressure (I also checked my tire pressure and they were ok). They said that because my front tires take 32 and my back tires take 29, that when they rotated my tires, it is now "reading" wrong. However, I do not think that the manufacturer would make a tps system that would could not handle a rotation. Now the problem is my tires are not maintaining air pressure well. Now firestone tells me that the reason the tps system is on, is because my tires are low in air. They tell me that due to the cold weather, my tires are retracting air pressure. They told me that the would replace all of my sensors for free. I have scheduled an appointment with firestone in a few days to have all my tire sensors replaced, and to figure-out why I have to keep putting or checking the air in my tires! Firestone tells me that there is nothing wrong with my tires, so why are they staying low on air? I called Sears and they think that Firestone borke a sensor of mine. I did see someone at Firestone beating really hard around the tire area. It is just really odd, because the tps light came on when my tire pressure was fine, but now it is still on, and my tire pressure is low! I dont' know if there is something wrong with my tires, my tps system, or what? What do you think? Firestone keeps telling me that there is nothing wrong with my tires and they are low on air due to the cold weather. However, I have had this car for nearly five years, and I don't remember ever going through this with low pressure and cold weather before with my tires?

Thank you,
Luci

PS: I feell like I am just getting a big run-around with Firestone. I keep taking my car in and they are telling me they have fixed the tps and my tires are ok, but 30 miles down the road the indicator (!) Light comes on again, and my tire pressure is low!