Your problem definitely sounds like it is your alarm module relay or immobiliser module relay, faults in either of these systems produces the same problems you have described, sometimes it can be both modules- for some reason they can go at the same time. you will find the problem will get progressively worse to the point every trip up the shop results in a no start, but then will have no problems for a week. is a frustrating scenario. people often look to starter motors, batteries, alternator, but it wont be as you are getting power to all accessories, and its not even clicking over, which is a tell tale sign. All lights on dash will come on as if its immobilised but no click when keys turned. The relays go after a while and build up a high resistance. If its just one module youll be looking at a few hundred with labour, if its both modules ( id reccommend doing both at same time) about $450. They will probably upgrade your immobiliser to a newer version that has a few different characteristics to the one you have now, i personally preffered the original system. Anyway thats what my money is on, im sure subaru will tell you the same after a diagnosis. let me know how it goes for my own curiosity. good luck..