How old is your battery, what voltage does it hold when the engine is not running, and have you cleaned up your electrical connections lately? Your symptoms point to low voltage in the electrical system. This could come from a weak charging system, loss in the electrical system due to corroded or loose connections, or a battery that is on its last legs.
I would start with the battery and the fuses if I were you. First, check your battery to make sure it is holding adequate charge (someone else will hopefully weigh in on what it needs to be) If the problem is not your battery, then clean off the contacts for any fuses (stock and the random aftermarket ones that POs tend to install), clean off your battery connections, and clean off the connections from the battery to ground. If the problem still persists, then check back with the results.
Camelman