Could be a software issue.

I upgraded my rc decoder to a rc3,... everything worked fine. The following season I set everything up the week before my first race. I had a similar problem.. there was about a 4 - 5 second delay after the car crossed the line... I hadn't changed anything... or so I thought. Something had changed in the OS (windows). Called the writer of my software (Autoscore), and he told me I needed an update... emailed me the update and everything worked fine.

Might want to check it out.