Uh?This was supposed to mean that the conversion problem that occurs when converting to degrees also would occur when converting to a matrix.But if I'd convert it into a rotation-matrix, I'd still get the problem at the edges, wouldn't I?

There are no conversion problems when converting quaternion to matrix (*) - did you mean from matrix to Euler angles? Then yes, whatever is the perceived problem with quaternion->euler (I am not sure what the problem is - are the values wrong? Why do you care that the values jump around a bit?) would probably crop up again.

Euler angles are usually terrible to work with (expensive, capricious and shall i say - bloody useless. IMMHO, aka YMMV), i would repeat the advice to use a quaternion and/or matrix where appropriate instead.

*) Quaternion transforms fairly easily into a neat equivalent rotation-only matrix (Orthonormal basis. So, just a bunch of unit length orthogonal axis vectors).

Perhaps useful for reference:

http://www.euclideanspace.com/maths/geometry/rotations/conversions/quaternionToEuler/index.htm

http://www.euclideanspace.com/maths/geometry/rotations/conversions/quaternionToMatrix/index.htm

http://www.euclideanspace.com/maths/geometry/rotations/conversions/matrixToEuler/index.htm