You could have a starter about to go bad... BUT, you should first check the wiring to the starter for corrosion. You need to do a voltage drop test, not a test for continuity. Very easy... just one lead of your voltmeter on the connection of the heavy wire at the starter; the other lead should be where the heavy starter cable ends near the battery. DISABLE THE IGNITION so the car will not start. Bump the starter and note the reading on the voltmeter... if more than 1 volt shows, you need new cable.