Good morning, David;
Your Minolta X-300 / X-370 problem sounds like the dreaded "intermittent" problem occasionally found in the electronics industry. We also call them "dogs," and for reasons you can surmise.
The real problem for the shop is that they may not know what was doing it, and it may not have done it for them while they had it, but in just doing things with the camera, it may have reconnected a marginal path in the wiring or contacts and now it is working. If the thing is not doing what it did to make the problem, it is very hard to find it.
And then there is the point that most of those things are really difficult to repair, if they can be repaired. In many cases, the repair or cure is to replace the specially designed proprietary circuit board with the parts and connectors inside the camera with another one. And that brings up another problem. When Sony bought the Minolta Camera Division, they made the corporate level decision not to support the older manually focusing cameras any more. What this translated to here in the United States is that everything in the warehouses at the old MinoltaUJSA in the way of parts and things for all of the old SR mount cameras was just thrown away. Gone. No more. If there is a problem with an old manually focusing camera that needs a part, sorry about that. Would you like to buy one of our new auto focusing Digital SLR cameras? A full frame sensor equivalent model is only $2,000 to $3,000 USD. Why, allowing for inflation, that is a bargain in comparison to what you paid for your old obsolete film camera. And look at all the new wonderful features you get with the new DSLR models.
Uh, huh.
So, we can still keep the old cameras going mechanically, but if they need electronics parts, it may not be possible, if another organ donor camera body cannot be found. If you have a reasonable investment in Minolta ROKKOR lenses, it is worthwhile to have one of the camera technicians perform a CLA with your camera body to keep that part of it going as long as you can.
Enjoy;
Ralph
Latte Land, Washington