I would do two things. First make absolutely sure your battery is fully charged. Second, check the voltage at the starter when cranking to see if you are getting the proper voltage there. If that voltage is dropped off a lot, you either have some wiring problems (corrosion in the wires from the battery) or the starter relay/solenoid is bad. Could also be the starter needs serviced with fresh bearings and brushes as well. I would expect there to be around 12 volts (or there about) at the starter when cranking. If not, you have to check where that voltage has gone. JMHO!