Random starter trouble

I have a random and really annoying problem with my starter. Sometimes it will just click when I turn the key, don't matter if its hot or cold. What I have to do then is to turn over the engine a little by hand to get it to dis-engage (dont know the correct english for this, hope you understand) and try again.

The thing is, the starter is relatively newly gone through by a company, and, when it will click it is also like I loose all the power in the electrical system. If I try immidiatley after it’s clicked nothing happens, the oil pressure indicator on the dash will go out and the key buzzer will go really weak. So, I dis-engage the negative ground on the battery and turn over the engine, put the negative back and then it will fire right up. This problem occurs totally randomly. I have checked the contacts on the starter, I have checked the contacts on the trans. I have ground from the battery to chassi, battery to engine and engine to chassi.

Any ideas? I guess it could be the starter after all, but is it not a little strange that I loose the power altogether after its clicked? The battery is fresh by the way.

It’s a 1970 Duster with a 65 273 Commando and 727.