Hi Daniel,
Very high magnification can darken an object. Saturn is a good example as you've found out.
Also, the quality of the "seeing" will determine how much magnification you can throw- the object will tend to shake and be hard to focus. All these things which you seem to be experiencing.
On very few nights can magnification be pushed that high to the limits of the scope.
Here's a guide as to how much magnification you can push per inch of apperture.
http://www.seqas.org/