Ya' know one thing that comes to mind for me is your sending unit location. The sender that inputs to my ECU is right at the head while the one that inputs to my dash gauge is on the water neck, just downstream of the T-stat looking at the combined water flow. They show between 15F and 25F difference, with the dash at 180F the temp at the heads, shown on the ECU calibrator will vary from 195 to 205 depending on load and driving conditions. With a 195 t-stat I was seeing temps up to 225 to 230F at the ECU, so I dropped back to a 180 stat. What temps (location) are you looking at? Also, have you shot it with an infra-red temp gun while at temp? That can give you an idea of water flow, and might show you an air pocket that's turning to steam?