Has it got a Bluefin on it? I put some old rolling road graphs of my car on the RSOC (done before i bought it) and this had a dip at about 4500 RPM and Shep from Sabre suggested it was something to do with the Bluefin and said he had seen it on other Bluefin cars. A later graph with Bluefin removed showed the dip had disappeared.
Money and time permitting i'll be getting it to either Sabre in Bristol or Jamsport in Northampton for a remap at some point hopefully.