A polydimethylsiloxane of high molecular weight is extruded through a two-dimensional die with high energy walls, by means of a controlled pressure device. In addition to flow rate measurements and flow visualization, a photo-elastic modulator set-up is used to measure stress birefringence in the flow locally and instantaneously. The same instabilities are observed as in capillary extrusion, i.e. extrudate cracking (shark-skin), upstream instability (melt-fracture) and wall slip with flow rate jumps. Birefringence measurements and flow visualization both show that the upstream instability starts before the macroscopic slip, with an asymmetric flow which develops close to the entrance plane of the die. It is shown precisely how the stresses vary locally when the macroscopic slip starts.