heyy mate,
mine did that when the battery was at about 11 volts charge,
the previous owner had the starter and ignition wiring replaced and made no difference,
so when I got the car, I put a relay in and the the relay basically switched the full battery load to both the starter and the solenoid, the car never has starter motor problems again :)
very easy to wire up, just need a high amperage relay, high amperage cable and a high amperage fuse(normal amperage of battery needed for cabling and relay),
1. disconect the wire from the ignition side of the solenoid and plug into either pin 86 or 85
2. attach high amperage cable to + side of battery and attach fuse,
3. attach other end of fuse to pin 30
4. attach pin 87 to solenoid on the starter motor
5. mount relay and tape up all connections
Note. DO NOT ignore the fuse, it could save your battery and all electrics in the car if something goes wrong :)