The P/S pump on the 3.0 engine is an absolute PITA to bleed. There is a lengthy procedure that you have to follow with the car turned off. If the engine is started before all of the air is bled then the the pump quickly foams all of the fluid and the system acts in the way you are currently experiencing, shudders and bad noises from the pump.
The aerated fluid also causes wear to the pump as it does not lubricate it properly.
Once the fluid has foamed you have to wait an hour or more for the fluid to return to normal before you can attempt to bleed it again.
Also it is quite common for the hose on top of the P/S pump to draw air into the system if not fitted correctly which can be hard to detect other than the P/S being impossible to bleed.
Course of action, either take it back to the service station and tell them to check all their connections and bleed it following the proper procedure. If you fancy you could try bleeding it yourself. Or you could take it to a Subaru dealer who have probably done this before but who will obviously charge you more.
I can't remember the specific procedure but it goes something like this.
Engine must be off and fluid must not be aerated (i.e. been off for a while ~1hour).
Turn the wheel lock to lock 30 times.
Let it sit for a little while.
Check fluid level.
Lock to lock 30 times again.
Check fluid level.
Then start it and really hope it doesn't foam again, if it does let it sit for another hour and repeat the process.
That isn't the actual procedure but it is something close to that.