 Each patient will participate in an open-label unblinded drug escalationtreatment run-in phase for up to 4 weeks until stable dose and frequency of amifampridine phosphate is achieved for 7 days. After this phase, blinded treatment effect will be assessed in a randomized fashion of continuation or cessation of drug Placebo starting with Period I duration 7 days. Following experimental Period 1, patients will be returned to the stable dose administered at the end of the open-label run-in period for approximately 2 weeks, followed by cross over treatment in Period 2 dosing for 7 days. After completion of Period 2, patients will be eligible for expanded access with restoration of open-label amifampridine phosphate at the same dose and frequency as established in the run in phase of the study. 