What is the fuel quality like?
If the FRP sensor is jumping around all over the place, I'd have a look at that 15-way connector that comes out of the rockerbox and make sure that thing's all gravy and not contaminated or loose or somethin. Assuming the FRP is under the valve cover like it is on a 6.4
You got all the other particulars to support it running? Proper RPM signal, cranking speed, airflow, etc?