R/C Tech Forums - EZ Lap System Issue

I just installed EZ lap transponders in our MiniZ cars and EZ Lap system in our home track. We have the following problem: the system records as expected but will fail to record an occasional lap. So everything works for 10 laps, then the transponder does not trigger the receiver module just for a single lap, then records several more laps, then misses a lap again. We have tested 4 new cars, 4 different transponders, no bodies on the car to interfere with the signal, all different locations under each receiver module and it happens with each car. We have tried multiple bridge heights and locations on the track. We have shielded the bridge from adjacent track without improvement. We have run only one car without improvement. We have tried various positions for the transponder (again with no car body on the chassis) and various positions of the receiver modules without improvement.
We are using Z Race timing software.
Can you provide any advice.