Does the same thing using the actual door switch too? Might be a flaky wire, might be the actuator going out. You could always swap the driver's side actuator to the passenger's side and see if it still does it. If it does its a wire, if not just get a new actuator, replace and dunzo.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.