So if I get what you are saying, you want to transmit every 400 ms, and get a response back. The DUT is spamming the bus currently around every ms. Since I have no clue what your DUT is, I don't know if this is standard operating for it, you may want to find out. For a write every 400ms, if you want to read data back, I suggest you look at the vi I posted. It will read all of the pending frames and return the data in an array. Then you can use the Arbitration ID you expect to recieve as a filter to find the frame you want. That way you don't read the data from one of the messages sent every ms instead.
Does this help?