Get the Engineers to check the CMC for any fault codes, try reloading the software, if no change, open up the forward section of the baggage bay floor and get the guys to look at the TCAS box under there ( I can not remember its name). Depending on which one is fitted there might be a LCD display on it, they need to watch what is displayed on it it maybe failing its BITE check and that will show up on the display. If it is failing you'll need a new box.
In earlier models pre phase 7 there was a way to do a TCAS confirmation check, however post phase 7 and the installation of the newer box this is no longer a option and the only choice is replacing it. Obviously check the connectors on it first.