The TFI ICM can be intermittent, as proven recently with my Bronco. Typically, symptoms are seen "when warm and under load". You will get the feeling that the ignition is not advancing properly causing stuttering, pinging and a significant loss of power.
SigEpBlue states as follows:
In my experience, probably the main contributing factor was a Distributor Mounted Gray "Push Start" ICM that was affected by increased engine heat from a new 408 Stroker. Remember that the ICM is heat sensitive which warrants additional cooling support. This motivated me to upgrade the entire secondary ignition system to include a Remote Mount Black "CCD" ICM with heatsink which my new MAF WAY1 PCM is looking for. All of my symptoms were subsequently eliminated.This is probably an addendum bit of information for diagnosis, but it is important: the most-common cause of ignition control module failure is bad spark plug wires and/or opens in the secondary ignition system. Almost every time I've replaced an ignition module, I've replaced the plugs, cap, rotor, and wires, if they hadn't been replaced recently.
The possible intermittent nature of a TFI ICM failure is also expressed when testing. The intermittent TFI can pass with flying colors, except after multiple consecutive tests which allow the ICM to heat up. This leads me to believe that many with chronic ignition problems, especially when warm and under load, actually have an intermittent TFI ICM issue.
If I were having intermittent ignition symptoms, I would immediately look at the TFI ICM. And, as SigEpBlue points out, complete replacement of worn ignition parts may be in order as well to minimize ignition system stress. Personally, I would only use the Motorcraft TFI ICM and I would ensure that it is the correct part for my application.
Many would also recommend replacing the PIP at the same time. In the case of a Distributor Mount ICM, Niehoff recommends to remove distributor cap and inspect connections where TFI module hooks to the Hall Effect Switch connector. If there is any sign of deterioration at the connector, the Hall Effect pickup must be replaced. Crankcase vapors are forced into the distributor due to excessive crankcase pressure due to a worn engine or an improperly maintained PCV system. The crankcase vapors cause deterioration of the insulation around the connectors causing the module to short out.
In summary, the TFI ICM is a sensitive critical electronic ignition component which can be stressed by heat, failing ignition components or crankcase vapors causing intermittent or complete ICM failure.
See this link. It is long, but has a lot of good information: Ford TFI Ignition Control Modules