if you are arent running a MSD distributor , pull your distributor out and put a pickup in it , and most parts stores can now test the TFI module for free ....tell them to run the test a few times to get it hot
Also check if there is heatsink grease on the back of the TFI module. I did not put any on when I got my MSD distributor, and I was having the same problem. Took the TFI module off, put some heat sink grease and no more problem.