@Federal
Yes, there can be many difficult to diagnose problems with a PC. Which is the reason I believe that you can't be a day-tech. You have to care enough about what you do; to always want to learn more about it.
Case in point: when I upgraded to an FX5900 video card, it was my first video card which required a separate power connection. I put the puppy in and my system wouldn't boot. It had be baffled for a while and a friend of mine suggested -- sigh unseen -- that I needed to replace my power supply.
This went against most of my troubleshooting knowledge and intuition. The PSU in question is a 440 watt "beast," and it's still humming away; powering another friend's PC to this day. What I realized after that encounter is that as PSUs age they won't necessarily be able to provide the same amount of power as they did when new. (That, or the PSU wasn't capable of its rated wattage from the very beginning, which was never an issue before that video card came into the equation.)
That PSU would've been marginally acceptable to power my PC w/ that video card, if it was capable for providing the rated wattage. While troubleshooting, I ignored my concerns over that issue and dismissed the PSU as being a possible cause. Lesson learned.
That's the problem of the day-techs. If you're not dedicated to your craft, you don't necessarily retain these lessons and probably don't seek out new information. In your case, I would've suspected the motherboard after the memory. Lesson learned; the hard drive is part of the "memory" equation, in that it stores much of the the data that ends up in RAM.
BTW, I'm programmer, not a hardware guy. I've learned to troubleshoot hardware issues out of necessity. However, I believe that anything worth doing, is worth doing right.
The lack of that kind of focus for someone's job is basically the problem with the poorly paid techs that work for such companies. It's not even the sorry pay. Everyone's gotta start out someplace; and anyone can find themselves in a tough spot where they accept a job just to make ends meet. Not caring about your work, however, is unacceptable.
There are many good reasons for being in that position, but we still must strive to do the best that we can, no matter the conditions. I've been in that situation in the past, where the situation at a job lead to a decline in my performance. I don't beat myself up about it, but I still strive for my ideal of reaching for excellence with everything I do. (That's the reason I usually preview comments a zillion times before I finally post them. And I still miss some errors, but it doesn't stop me from caring enough to always try get it just right.)