I can sympathize with what you are facing there. This November here where I live has been the coldest anybody can remember. We have already had a total of almost three feet of snow in the past three weeks and many nights below 0F. Looks like more on the way. Certainly not good weather to be working in the garage unless you have heat.
I guess I had not realized you still had your old IP available. If so, installing it is a great way to determine whether the current replacement IP is responsible for the trouble!
Did the engine run in a satisfactory way with that original IP? One thing I cannot recall (apologize if you already told us earlier) is what was your reason for removing this engine for repairs in the first place. Was it just proactive reconditioning? Or did it have operational issues as well? If the latter is the case, then installing the old IP may not be as conclusive of a step.