[Mart]

Might I suggest it's time to get out and get under that automobile.

Jack the car up and support it safely on stands. Pull the starter. Closely inspect the starter bendix, teeth, bearings, anything that might not look 100%.

Closely inspect the teeth on the ring gear. Put a mark on the last one you looked at. Turn the engine and inspect the next little bit and put another mark on the last tooth. Repeat until you see the first mark coming back around.

Hopefully after doing all that you should see something not quite right and might be in a position to fix it.

Got a spare starter? If so swap that on and see if the behaviour changes.

As the problem is intermittent, there might be an argument to suggest it is the ring gear rather than the starter. Only a close inspection of the parts involved will show what's up.

Mart.