It's not the starter moter per se, they generally work, or don't...now, if it cranks slowly, then it might be the starter motor, but it's more likely in the engine control electronics, crank angle sensor going out, ignition lock, something like that. Since it starts, it's going to be hard to trace down...
2006 Corolla (kid's car - 108K, Conti Eco Contacts, Castrol Edge 5W30)
2005 MB S600 (63K, M1 0W40)
2002 V70-XC (165K, Nokian WR G2, Pennzoil Ultra 5W30)
2002 V70-T5 (143K, IPD bars, Bilsteins, Extreme Contact DWS, Mobil 1 5W40)
1992 MB 300E (Sportline - 123K, Nokian WR G2, Edge 5W40)
1990 4-Runner (V6, 4WD, mild lift - 239K, 31" BFG AT KO, Edge 10W30)
1932 Packard Sedan (straight 8, dual sidemount spares, original paint and interior, Shell Rotella 15W40)