TPMS replaced and still has problems

For the past year, the Tire Pressure Monitoring System has been randomly saying one of the tires needs the pressure looked at. The warning light flashes for a particular tire. I had the sensor replaced 3 times at a well known national chain and it still flashes off and on at times. Sometimes it will flash that a different tire has a bad sensor. The national chain mechanic showed me where on the machine it says all the sensors are okay. When I took the van to the Chrysler dealership, he looked at the codes and suggested all 4 sensors be replaced for a cost of $650-$700 even though the warning indicator lights said one sensor was bad. Needless to say, I didn't have all 4 replaced since I felt they weren't very confident this would fix the problem. So I just live with the warning lights randomly flashing when they feel like it. Some days they don't flash at all, and other days the light flashes. I just make sure to regularly check the tire pressure, especially when I'm going out of town.