Since ALL of your starters (original and 2 replacements) have essentially the same problem it doesn't seem like a starter issue. Remove the starter at the spot where it disengages and examin the ring gear on the flywheel. If that looks ok then start checking out the wireing. Hopefully you don't have a broken tooth on the flywheel. That's an expensive repair. If you do need a new flywheel though also have the rear engine seal replaced. You can also try manually turning the engine over slightly after it disengages. If it then catches you probably have a bad flywheel. You can also try connecting the starter directly to the battery with some jumper cables but that can be a little tricky.

I was taught to take a voltmeter and test for voltage at each connection between the battery and the starter, including testing the ground side of the circuit. Voltage difference should not change between the battery and the starter; if it does, you've got a dirty connection or a bad wire.

Even if you didn't have chipped ring gear (gear on the flywheel) teeth to start off, you may by now with all that grinding away - I don't know how easy it is to observe them, but I suppose you can remove the starter to inspect them if you don't have any other way to do it. That seems unlikely to have been the original problem, though.

How do you know it's the starter and not some teeth missing from the ring gear on the flywheel?

I don't quite agree with Klaus - more voltage is always better but the starter has such a heavy current draw that it's common for the voltage to drop as low as 8 or 9 volts during starting (especially in the cold or if the battery is not brand new). The starter, solenoid, ignition and fuel systems are tolerant of this loss - otherwise nobody's car would start in a northern winter. If you don't believe me measure the starting voltage on one or 2 other cars that start normally.

But maybe there is a loose connection or bad ground and the 10 volts you measure drops much lower (like 5 or 6) from time to time (not while you are measuring it).

If the starter is only getting 10 volts, that is a real problem. Check the voltage from the line from the battery and verify 12 volts. You may have a bad wire or dirty contact at the battery.

Klaus

Sounds as if you need a good used starter. Not expensive or too hard to change. Did one on son's 94 850 with similar problem.

dick