starter motor issues...

Assuming the starter is good it's most likely a voltage drop in the wiring and or switches. Add a relay to the starter circuit to take the load off the old wiring and switches and give the starter good clean voltage.

Check the leads going to it wriggle them sometimes they have a break in the wire from heat ,seen a ford used to do a disco when ever it rained and wouldn't start it was a crack in the earth lead the big one ,other then that my guess is soliniod on the starter is full off dust etc.

Are these clean?

I smother them in grease which prevents the problem from recurring ..

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Take it to an auto electrician.
Sounds like you are spending money chasing your tail: two starters plus a battery.
A sparky will have the right gear and know how to diagnose the problem.
As David said above, a relay in the starter circuit can make a big difference.
David

if its the original starter the new replacement will be smaller, wrap some of that heat shield wrap around the extractors where its close to the starter.

What's the CCA rating of the battery?

Check the starter by bridging between the +ve and the solenoid connection.

If it works reliably I would consider the Ignition switch to be faulty..

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Test as T has suggested, and if it works ok, then seriously consider running a new power lead direct from battery to fuse box.
Had the same problem on a HZ Tonner and the Auto Lec fixed it this way.
Another way is fit a relay to the starter so it gets full power when starting.
Just realized relay was the first suggestion from hq308.
The wiring on old cars can be a problem.

Sounds like the copper switch contact plates in the solenoid are flogging around and causing intermittent and varied results.

pull it apart what was wrong with it ? did heat kill it or is it all right?

Wow
Someone said thank you
There are some good people on here to help a fellow members out in ther time of needs

Fingers cross it’s sorted out mate