I own a 2003 Saab 9-5, 90,000 miles in very good condition. It starts reliably every morning (when cold) but after driving to a store or to work, the car will not start - won''t even turn over - and I often have to call for a ride home and return to my car several hours later when the car has cooled down. I had the ignition switch replaced at someone''s recommendation, but the problem persists. When it doesn''t start it truly does not even TRY. Accesories work, radio comes on, etc but the engine does not crank at all. Please give me your best recommendation about where to go next. Different mechanics have said I should check the camshaft position sensor, or the transmission range sensor (?), and others have advised that I replace the starter. What do you think I should try next??

Optional Information:
2003 Saab 9-5 4cyl

Already Tried:
Ignition connections, ignition switch replaced, battery cables checked, battery strong, ??? Can't think of what else. This has been happening for 3 or 4 months. It doesn't happen EVERY TIME, which makes it even more frustrating. But I've gotten to the point that I don't want to turn the car off when I stop for gas because it might not start again. Last week I had to leave the car and get a ride home from the supermarket. Well, the car was towed and the episode ended up costing me $300. I can't afford that!