I bet you have some electrical cabling under the concrete floor around the loop area. I have seen this issue on outdoor onroad tracks. They setup a track, and the electrical for parking lot lights, a sign that is out front of the property, or the electrical coming into the building is near the loop and cabling.

We make our own loop, and only have issues when the solder joint breaks, or when people dont lay the transponders flat in the chassis. MyLaps original documentation (RC2) says do not put the transponder on the servo...