Battery is most likely low. Have it checked to verify.
August, 23, 2012 AT 3:34 AM
Voltage drop across the ignition switch / starter wire circuit?
The next time she does not crank... lift the rear seat, disconnect the
thick red wire from the connector and using a jumper wire, touch it to
the battery + post.
If she cranks every time you touch the wire to battery +, but does
not crank when connected to stock wire harness... then you might
have too much of a voltage drop across the ignition switch.
Attach a voltmeter to the red wire to see how many volts are
at that connector when you turn the key to "crank".
Anything less than 9/10V is no good.
Instead of an ignition switch, you can slice a relay in between
that connector under the seat... to supply battery + power "direct"
from the battery to the starter wire, when the key is turned to crank.
The low voltage from the ignition switch "triggers" the relay, which
then sends full battery power to the starter.