Starter intermittantly doesn't work

Sometimes I take the starter motor off the vehicle. Lay it on the ground, hook up jumper cables properly from battery to starter motor, it should do two things, kick out the drive gear and spin. Do you have a flywheel cover that you can remove and watch the starter drive gear engage the flywheel? Also, check the flywheel teeth.

Maybe problem with starter motor? It does two things, it kicks out the starter drive gear and spins. The flywheel usually has cover, remove the cover, try to see if starter drive gear engages the flywheel teeth, with key in the crank position.
Some of the national brand auto stores will test the starter for you. Sometimes, I take the starter off, lay it on the ground, hook up jumper cables properly from battery to starter motor. It should kick out the drive gear and spin, granted that is no load test. Intermittent issues can be a pain.
I just want to add, when working under a vehicle, safety is always your responsibility.

I just wanted to add, make sure battery has a full charge and battery connections all good.

It sounds like the starter solenoid is going bad on your starter. I would remove it and take it somewhere to have the starter tested

I was having the same issue with my 95 GMC. I replaced the battery and alternator. It would start fine at times, at other times it would be sluggish. I finally took the starter off and had it checked. Turns out the starter was bad. When it would get hot, it would act as though the battery was dead. Replaced the starter and it has not acted up again. Have the starter checked. Replace if bad.

It's not a fuse because when they go bad that circuit is dead and won't work at all...not intermittently. It may be the beginning of a failing starter. The starter heats up and won't restart until it cools down. Also the fuel pump can act up in the same way.

you starter had developed a dead spot, check to see if you have batt signal to starter when this happens, if yoy do then its the starter

Take the starter cover off, look at the Plastic/metal gear. Is it in the up position on the flywheel, if so, the starter is not operating as it should. The gear should come back down out of the flywheel area. When the gears stay up like this, it causes the flywheel to be in a bind. The starter has to have the bendix to shift the gear up into the flywheel to turn over the engine. This is a common problem when a mower will start sometimes and hesitate like it don't have enough of battery power to turn it over, but in reality, it's binding and will burn the starter up if you try to start it too much.

Probably when the dealer is testing it is fine because the problem is intermittent. There may be a dead spot in the starter so when it hits that spot it acts up but other times its not on that spot its fine. Replace your starter and you will probably get rid of the problem.

Take to Autozone or Oreillys for a starter draw test. Sometimes the starter will have a bad place and will not energize to spin the starter.
Another possibility is the Throttle Position Sensor which is located on the airhorn. This balances the fuel and spark systems for the gas pedal. When you start the engine it gives you the base idle and must work with the cold start system.