It has occurred to me that I didn't take the solenoid to bits and look at the contacts - perhaps if dirty, they'd be causing resistance thus reduce current to the starter motor.
Voltmeter connected across the large solenoid contacts should read zero (less than half a volt) when you are cranking. Only do the test if its to hard to replace and you want to double check.
The main culprits for this is either a worn commutator which causes excess sparking and therefore brush wear or insulation failing on the windings - which if serious will call a halt to the complete motor anyway.
How are you getting on with this?
John
There's a possibility that the starter cables may have some resistance....corrosion and so on. The battery to bodyshell and bodyshell to engine block cables would also have to be perfect.
All these things become cumulative after a while, and of course the battery and alternator output could be slightly down on new.
John