We run into this a lot. A car that's been from shop to shop in the area, chasing a problem nobody seems able to fix. As I said before, we're a small shop (2 bays, one dedicated to NY State Vehicle Inspections). We work by appointment only because we have to plan our time wisely. We also charge a little more because we're good and we know it. We have to be due to the lack of space. Shit can't sit around while we're thinking about what's wrong with it. Cars get diagnosed quickly, parts are ordered, the car is fixed, and it leaves. Most (95%) don't stay overnight.
That said, we get people trying to save money (we're not anywhere near dealer prices and we work fast so our labor comes out less anyway) or time (they can't wait a couple days), taking their cars to other places. Especially true with a service (notice I don't call it an 'oil change') which will set you back $50 in my place. We get those cars back eventually, with a buncha new parts under the hood, with the same problem (so how much did you save getting $19.95 oil changes). We're not parts-changers. We treat a customer's money as if it were our own and they get only what they need.
If it's an intermittent problem and I can't replicate it on command, and I'm not sure of what's causing it, I'll tell the customer that, and ask them to put up with it until it becomes a bit more regular (nothing fixes itself and it's gonna get worse). I'm not gonna throw a few hundred bucks in parts at it, charge you another couple hundred bucks in labor, on a 'maybe it might work'. Odds are, if I can't find it, nobody else will either, with the exception of the dealership. Those guys see the same cars all day long and they know about the little crazy shit that is chronic to the brand. I have, on occasion, advised a customer to have a dealer look at it rather than take their money to do something I'm not sure of.
If you have a shop that does timely repairs at a reasonable rate, stick with them. As I always say, there should be no surprises when it's time for you to pick up your car, and your car shouldn't be sitting at the shop for weeks, 'waiting for parts'. If you live near a major metropolitan area, most parts (brakes, starters, alternators, batteries) can be had within an hour or two. Special order usually takes a couple business days. Cars sitting around in various states of repair don't make me money, neither do pissed off customers who paid a big bill a couple days ago and the problem remains. If your guy required several visits (with requisite parts and labor charges each time) to fix a problem, look for somebody else.
I'm very happy with being known as the Last Chance Garage. It means I'm doing something right and the appointment book stays full.