I'd have to rethink my starter diagnosis. It IS a very common problem with certain starters, but you should hear one kind of loud clunk when it engages but doesn't spin.
Since you noticed moving the vehicle helps, is it possible the same thing could be accomplished by just moving the shifter? Try cranking it in neutral or by just moving it in and out of park a few times. If either of those things help, it's time to look at the transmission range selector. That's a fancy name for the neutral safety switch.
If moving the shifter doesn't help, the next time you catch it acting up, remove the plastic cover from the starter relay and try to crank the engine by gently squeezing the contact. If it cranks that way, (be sure its in park), that proves the starter itself and the wiring are ok. If it fails to crank even once, suspect a gummed up shaft that the starter drive slides on to engage the engine. You can do this multiple times if you leave the ignition switch off.
Also, with that cover off the relay, watch for the contact to move when a helper turns the ignition switch to "crank". If it doesn't click, you will need a test light or cheap digital voltmeter, and I'll give you the steps to take. If possible to catch this acting up in a convenient location, try to keep it in that bad state so a defect will show up during testing. Some people will poke and pry until the dead circuit works, then think there's something to find through testing.
Thursday, March 4th, 2010 AT 4:25 PM