Zahos: only other thing, is it could be a slipping drive... you know when you start an old ford and the starter will spin, but will not turn the motor over. Like it will whiz away but nothing happens? the drives do wear out too, they have a rachet inside them that will wear out and not lock properly. Best bet would be to get someone to rebuilt it with, new bushes (could be bearings, I have not seen inside an R33 starter - but I doubt it), drive and a set of brushes. Your heart will be at rest then if the sqeualing doesnt stop, you'll know to spend money elsewhere.
Where I work we cannot diagnose intermittent working drives in our test bench, as we cannot load them up as much as an engine would, we have to listen to them on the car and say what the problem would most likely be.
Sumo