I know some people would disagree, but when it comes to cars, it's best if you do what I tell you. Case in point: Intermittent problems.
These are things that go wrong with your car whenever they damn well please, but will go away for an indeterminate amount of time. In today's computer-controlled cars, shit like that happens. These are the hardest things to diagnose; the things that I can't make happen when I want them to. If you tell me your car bucks like a rodeo horse every time you go uphill it's one thing. I can take it for a road test and feel what it's doing and troubleshoot it.
If your car, once every couple weeks or a month, decides to spit and fart for 10 seconds as you're driving down the highway, that's another. If I can't find something stored in the vehicle's computer, or recreate the condition, there ain't much I can do. This is when people look at me like I'm an idiot when I tell them to drive it until it happens on a regular basis, or under the same conditions (weather, road, speed), and then bring it back. "Don't you know what it is?" They ask me.
Now, here I have two options. I can tell you the truth. "Not really. I checked a few things that might cause the condition, but I can't find anything out of order."
Or lie. "It's a computer problem. Leave it and I'll call you when it's done."
I'd rather tell the truth than throw parts at a car (usually at great cost to the customer), and have it not fix the problem. This doesn't satisfy people for some reason. They want to spend money. Fine, take it to somebody else. Some do and it isn't long before the same car is back for the same problem. This time it began happening regularly and I was able to diagnose it. Thing is, even though they don't tell me they took it to somebody else, I look under the hood and see new spark plug wires, spark plugs, filters, every fucking sensor on the engine, and belts. They took it to somebody who told them a lie, soaked them for hundreds, if not a grand or more, and still didn't fix the problem.
So, if you have a problem that comes and goes, without a discernable cause, and your mechanic says he can't find it right away, don't look at him as if he's a moron, or run to somebody else. Consider it a good sign that you have an honest guy.
Note: This shouldn't be applied to safety problems. As in, every so often my car swerves wildly to either side. There's something seriously wrong and your mechanic should be able to diagnose it easily. This is for stupid shit, like the radio not working at odd times, or the heater shutting off, a spit and a fart once in a while, shit like that. Brakes, lights, tires, and steering are all critical and should be attended to as soon as you notice a problem. If you do regular maintenance at a reputable shop (oil change and service EVERY 3000 MILES), your guy should catch little problems before they turn into big ones.