Starter won't turn or turns intermittantly on your 993?

Ok so here is my understanding in the order of testing and parts replacement:

• Check the battery and that it is producing greater than 12.5 volts. If not charge it;

Then;

• Check or simply replace relay R61 on Tiptronic or cars equipped with the immobilizer feature, this is not the DME relay.

Then;

• If the solenoid/starter ground strap looks good and well connected and you see no corrosion on either end

And the;

• Battery connections are tight look clean and show no corrosion, the negative at both ends.

And;

• The heavy hot wire on the solenoid/starter looks good and well connected is getting constant power and you see no corrosion

And the;

• Switched wire - the thinner positive wire going to the solenoid/starter is seeing power when the ignition key is turned to the start position with the clutch pressed to the floor and it looks clean with no corrosion; 
• If no power check the safety interlock switch on the clutch peddle, if your car has this feature, to verify it is working properly;
• Then if no power test or just replace the switch on the back of the ignition key mechanism in the dash.

Then; 

• The solenoid is physically stuck internally to its housing and needs to be lubed or replaced

Or if it is not stuck;

• The switch internal to the solenoid is not making a connection and needs to be cleaned or the solenoid replaced.

Or if the switch looks good and seems it is working then;

• The motor's brushes, commutator, windings or Bendix (gear inserter) are bad,  and unfortunately, these items internal to the starter are outside the scope of what most folks can fix DIY,

And;

Probably the starter needs to be replaced

Rebuilt starters can widely differ in their service life based on 

• The condition of the core they were built from, 
• How their components were tested and rebuilt.

Wire connections that are part of the starter circuit can be corroded internally while outwardly looking reasonably ok. For this reason some; 

• Disconnect them, clean them, and 
• Check the wire filaments on the end terminations and reattach them to be sure.

I believe that is all there is to a starter system. There is nothing special about how the design or how the starter works in a 993.

Tiptronic equipped cars have a slightly different looking starter. Getting to the starter without a lift and the proper tools can be a pain in the butt.
Andy