My PC Works at the Repair Shop but Not at Home – What Gives?

Hey everyone,

I'm really at my wit's end here. I built my PC about two years ago, and it was running smoothly until a couple of weeks ago when it started acting up. It was intermittently refusing to start, and now it won't power up at all when I press the button on the case. I get this brief flash of LED lights, but then nothing happens – no fans, no lights, total silence. Here's what I'm working with:
- Aorus B550 Elite V2 motherboard
- Corsair 2x8 DDR4 RAM
- Ryzen 5 5500 CPU
- RTX 3060 12GB GPU
- Thermaltake 650W 80+ Bronze power supply
- Thermaltake S200 TG White case

To troubleshoot, I took it to a repair shop, but of course, it powered up perfectly for them. They suggested it might be a power issue since they let it sit unplugged for two days before working on it. When I got it back home, it started once and I used it through the night. But the next day, it wouldn't start again! I even tried taking it back to the shop a second time, and it turned on right in front of me.

Here's what I've already tried:
- Resetting the BIOS by removing the CMOS battery (no luck)
- Buying a new power supply (still didn't work)
- Testing it outside of the case with just the motherboard, CPU, and one stick of RAM (no success)
- Blowing into all connectors (desperate measures, right?)

I'm really hoping someone out there has had a similar problem or might have some ideas on what else I can try. I'm getting pretty desperate here!