F53 and code P1299

Ford V10 chassis engine temperature gauge shoots off the chart, but not actually over heating. OBD2 shows temp at normal but throwing code P1299 cylinder head temperature overheating. Pull over reset code and all is good for 15 minutes to a hour and then temp gauge shoots up and engine goes into “limp home mode”. Mechanic did some research and told us that it was cause by misfire so we changed and plugs. Keeps happening, so we replace all the coils. Still happening, so replaced cylinder head temperature sensor and still happening.
Anyone have same issue and find a solution? Next step is changing the coolant crossover pipe, but that looks like a pretty big job and would like to see if anyone has found better solution.