When troubleshooting a problem on anything, not just cars, intermittent failures are by far the biggest pain in the chair.
In fact, dealing with intermittents quickly separates the men from the boys and the women from the girls. Taking an intermittent problem in a car to most mechanics at $100/hr is a real good way to waste a lot of money, just as the average tech in the electronic factory where I worked had no chance to work out an intermittent.
When my 2002 Sienna, now parked, was fairly new, I got an intermittent evap failure. Sometimes it would work for many weeks, then fail steadily for a few weeks. I made notes and kept searching the Internet. I did NOT take it to a mechanic. I was aware there might be a mechanic who had learned what causes that problem, but it was my money and I was not giving up easily. I know how furious I would be if I took it in and got a bill for several hundred dollars with a NTF note on the bill. I can do that for free.
Finally, a man on Sienna Chat reported that he had the same failure on one of his two Siennas, and he swapped stuff until it followed the canister assembly when he swapped it. He said it looked like there were self-actuating components there, and they must have been getting sticky. I took it to Toyota in Pharr, signed the waiver that said I was responsible if it didn't fix it, and it has given no more problems at all.
At that time, when I posted that tale here, one of the mechanics commented if a customer came in and told him what to replace, he'd hand him the tools and tell him to do it himself. In my experience, techs with that sort of attitude were ALWAYS very poor diagnosticians.
That of course is a mechanic's decision to make. But, after 31 years as a diagnostician, I do not feel a need to apologize for making the diagnostic decision myself. I worked for some years on 'black boxes' (they were actually gray) that had a standard cost of $500,000 USD, due to very low quantity. (As I typed that dollar number, I hesitated; that is a lot of money for one box. I thought again, and yes, that is what they told me.)
This issue of intermittents came up again in recent times. We bought a new MABE refrigerator. I think Mabe makes a lot of refrigerators for other brand names in the US and Canada, but sells them as Mabe here in Mexico.
From the time it was new, it has had a strange problem. For weeks it will maintain temperature fine. Then, one day it either starts freezing foods in the main cabinet, or lets it warm up to around 50 degrees. If I readjust the controls, it will run fine again for weeks, then goes the other way. Usually, it's just a nuisance, but I have been sick and needed yogurt for medicine. Letting yogurt get too hot kills it.
So we called a repairman, the same one who earlier replaced the compressor. But, the compressor was not intermittent and he handled it well.
I told him what we had seen and it soon became apparent he was the lower echelon of diagnosticians who MUST SEE IT FAIL HIMSELF! Pathetic. He hinted he knew it had to be in the control assembly inside the main box, but would not replace it. I was more than willing to pay for a new control assembly, just to see if we can fix it. I realize a lot of car customers who know nothing about diagnostics will be furious if you replace something with a NO-FIX, but I worked on customer repairs for over 31 years. And, I well know that in some cases on an intermittent failure, you simply have to eventually try the most probable component.
He instructed me not to touch the controls, but to call him when it took off again. A few weeks later, it ran up to 50 degrees again. My yogurt was at risk, so I cranked it down again. It would be several days before he came, and it's an all day trip to buy more Sugar free yogurt.
I decided I was not going to do any more business with him, at least not on this problem. Just how much suffering will I experience while waiting for him to be satisfied that he has seen the failure himself?
By a strange quirk, the temperature has been almost perfect, around 38 degrees for weeks! So, I am guessing there is a mechanical binding or something in that control and we just happened to twist it just right.
If it starts failing again, we will shop for a new fridge, and ask the cousin who is a dynamiter to blow it all over the hillside... Just kidding, of course, but wouldn't that be sweet?