Epilepsy
You suffer from severe epilepsy. You are subject to seizures during which your limbs tremble uncontrollably and you cannot speak or think clearly.
Make a HT roll whenever you are in a stressful situation (especially if your life or the life of a friend is threatened). If you have any sort of Phobia, exposure to the object of your fear counts as a stressful situation; roll vs. HT once every 10 minutes. On a failure, you suffer a seizure that lasts 1d minutes and costs you 1d FP. You can do nothing during that time.
You may attempt to induce a seizure through autohypnosis. This requires one minute of concentration and a successful Will or Autohypnosis roll. Seizures near areas charged with supernatural energies might produce visions. Whether these are useful is up to the GM.
Low-tech individuals who do not understand "fits" may be awed by them, and perhaps even believe your seizure represents a communication from the gods. Make a reaction roll at +1. A reaction of "Very Good" or better indicates religious awe! "Poor" or worse causes the observers to flee – not to attack (unless they had other provocation).