One way to verify a bad TFI is to use a Timming Light. They will on occasion scatter timming when they start to fail. Rev it up and watch if timming is jummping around. It should stay fairly steady. Also can check with Lab Scope if you have one. Keep in mind that the Hall-effect in the distributor can also fail causing similar condition. You can save yourself from future TFI failures buy relocating it away from the engine heat and vibration. I learned that from a Ford engineer. You just have to make a harness from the Hall-effect to the module.

Good Luck.