Overloads can fail over time, but it doesn't explain the fuse. Motors don't always draw the same current on all phases, so B might be the high phase, or the B phase is failing to ground somewhere.
To test the starter, check the voltage from Line 1 to Terminal 1 and Line 2 to Terminal 2, etc. If you have a problem in the starter, one leg will read higher than the others. I would expect less than 1 volt, but comparing the readings on the three phases will give you an idea if something is out of balance. If you have excessive voltage drop on one leg, you can move your probes and narrow down the source of the problem. Then, try the disconnect.
More likely, though, something is changing on the load. Maybe a solenoid sticking?