I'm joining the party late so sorry about the long post. I think most all of you are on the right track. If I was giving this guy advise at the shop and couldn't get it to act up I'd say look at a fuel issue, pump and / or clogged filter also possibly a clogged sock in the tank and / or rust in tank. But could be a coil too.
I'll tell you as a pro, in the old days there we had a wall of books. Then we got a case of computer disks, then online manuals which we still use today, but we use youtube, and forum searches the most. We call it YouTube academy lol. Both for newer and older cars. There is information out there on the internet no factory manual has.
I disagree with multiple filters. I was just inside my 37 year old fuel tank and it is clean as a whistle. I have one filter only, if your tank is so bad you need more filters just replace the tank, there only a little over $100 each. Each filter creates a restriction, even when clean and as they collect junk, they just get worse. I once had a mid-70's dodge van in the shop, it was starving for fuel on long grades and he had vapor lock issues in anything over 75F weather. Turned out someone had installed 3 fuel filters on it. One at the tank, one mid-way (which was the factory spot), and one before the carb. His tank was ok, so we eliminated 2 of those filters and it stopped acting up on him.
Labor rates can vary dramatically depending on where your at. In my area most independent shops are $110 - $150. A mid grade ICM, is $25 - $30 my cost, so selling that to a customer at $58 is quite reasonable. 1 hour for diagnostic is pretty standard. $132 to install the ICM, so that looks to be a little more than one hour of labor, but perhaps they are setting the timing with that. Perhaps they would be in and out in 1/2 an hour and give you a break on that too. Everything else adds up and they cut you diag charge since they couldn't get it to act up. All of that points to them being a legitimate place. And everything they did was for $58. I'm not saying you should take it back to them, doing work yourself can be very rewarding, but I wouldn't be afraid to use that shop based on what you have said.
I couldn't agree more. Vacuum leaks are at least tangible and in a shop environment usually not to hard to find with a smoke tester. Plus there is always the replace all vacuum line method. Grounds, those are a real nightmare. I once had a 1994 Buick Century at the shop. It would stall on the customer when coming to a stop, then restart, would happen maybe once a month. It progressively got worse over the course of a year or more, sometimes it would take several minutes before it would restart. At the shop everything would test out ok, and we could never get it to happen for us. It got to the point where if it happened in town, I'd ask her to call us, I'd drop everything and hope to get to her before it would restart. Eventually I was able to make it, before it restarted. No fuel pressure (in tank electric), once it restarted we drove it to the shop. Replaced the fuel pump thinking it was a intermittent issue with the pump, only to have it happen again. On that failure I found a loose ground connection inside the trunk.