Just to put it in perspective, one needs to troubleshoot to really sort these things out. Cannot without tools and hardware to troubleshoot with. PC shops are lazy. I have enough pc's at home, so if I have a problem with one, I can easily use another ones power supply. But then again, half the time I got work spares around, so no need.
Most of the time when we encounter such weird problems I first test with a power supply, and 80% of the time it is that. PC shops are exactly that: PC shops. What does it benefit them to fix your pc for you without using spares? They benefit from sales, so all their advice would be motivated by sales. And a motherboard is more expensive than a power supply. As an IT support tech, I always start with the cheapest part in troubleshooting, and work up to the expensive parts. But usually there is very little middle ground...it's either power supply or motherboard. Other things don't die nearly as easily as these, unless they are abused.