Bob is dead on the money on this one. When I worked on motorcycles, I probably replaced at least 50 starter solenoids every spring rush- always the exact described symptom. The contacts inside the solenoid which is a magnetic switch will arc a bit during operation, eventually pitting up the contacts. Like Bob said with some of them you can be bold and take it apart and clean the contacts, with others (such as most on Japanese motorcycles and newer equipment) you end up tossing it in the trash and replacing it.
The starting system is a bit different on SLT1554's depending if it's a Command or Courage engine in it.
-Fordlords-