Randomly will not start

That test is irrelevant because there is no load on the starter. It will typically draw about 250 amps to get started, then once it is spinning, it acts like a generator and develops "back EMF" just like a generator develops a voltage. There is no way to measure that current but we DO see the effect it has when it opposes battery current. That causes battery current to immediately drop to around 150 amps.

When you do this testing off the engine, the starter motor is free to spin way faster than normal so it develops a lot more voltage, (that back electromotive force) and current goes down even more. Big old heavy V-8 starters will draw less than 50 amps under those conditions and appear to work fine. That's why the only way to accurately test a starter is on the engine with a load. For a better explanation with sad drawings, see:

http://randysrepairshop.net/starter-systems.html

At the bottom is a link to Ford starter system diagnostics. There are four different systems they used over the years. One of the things you might especially look for is a corroded cable at the solenoid, battery and starter. The strands of wire corrode away under the end of the insulation where you can't see it.

Another good test with this type of problem in a high-current circuit is the "voltage drop" test. It works better than measuring voltage at various places in the circuit because when the starter is kicking in and out rapidly like yours is doing, the voltage AT a point in the circuit is bouncing around wildly due to those voltage drops ACROSS a point and digital voltmeters do not average out the fluctuating voltage to make it readable. Here's a link to explain voltage drop readings and why they work better at finding this problem:

http://randysrepairshop.net/voltage-drop-tests-in-a-high-current-circuit.html

Yes. Perform 2 tests:
1:Starter draw. This tests how many amps the starter requires to spin.
2: Battery load test. This tests if the battery is capable of supplying the type of current needed to start the vehicle, and if so, at what voltage. You need to know the cold cranking amps rating of the battery. It should be printed in the label somewhere.

Just for giggles

Completely remove your battery and have it tested at an auto parts store, their machines do a thorough job.12 v on a voltmeter does not load test your battery, so your home test may not be good enough!

If you feel you can't do that. At least disconnect your battery for 10 minutes then hook it back up. This may reset your computer when it senses a full 12 volts

In my pic below---this '85 cj 7 had a new battery in it, it would not crank, unless it was jumped

This connection was at the battery----the other ends of the wires may be this way also. Be thorough! Even loosening and "scooching" the connectors around may make better connections

The medic