Starter "flat spot" - what's causing this?

[Radek]

Hello everyone;

For quite a few months now, my 1988 90Q has suffered from this intermittent
problem.  Sometimes the car starts like a champ, other times you turn the
key and ... silence.   There is power draw (voltage drops) but nothing
moves, clicks or otherwise.  Total silence.  Push start.

The starter was rebuilt a couple of years ago.  So far, I've replaced the
ignition switch, put in a new battery and a new battery ground cable.  Still
the same.  I've developed a habit of parking at elevated spots with a clear
line ahead.  However, winter is coming and having a starter that works 100%
of the time would be nice.

As a note, I found one way of dealing with the issue:
When the starter wouldn't work, I put the car into second gear and rock it,
trying to get the engine to rotate forward.  After such treatment it would
often start.  However, a strange, grinding noise persists for a few seconds
after the engine has started and then something disengages, everything
returns to normal.
Any clues?
Thanks.

Radek
88 90Q
91 V8Q 5-sp.