Huh. Is that line on the scope a one-time glitch at power up, or does it happen on every cycle?
"Rummaging through my old parts box unearthed a power supply that was a bit more powerful than the one that came with the router, and even fit the connector." Is he describing a "wall wart" type power supply? A picture would help. A teardown would help more. There are large numbers of really crappy wall-powered DC supplies available, mostly from China.[1]
Look for a UL approval marking and a UL approval number. That indicates at least some attention to quality. UL is mostly concerned about fire safety, but their basic test for power supplies includes connecting it to a load box at the power supply's maximum rating, then running it for a while to see if it overheats. This catches power supplies with exaggerated ratings.
Avoid no-name power supplies. This is a well-known headache.
I've found that a lot of devices anymore just pack in those no-name power supplies. Many things don't even ship with branded PSUs now. It makes sorting my box-o-wallwarts a giant pain in the ass.
This is the opposite of my strategy: each wall wart in my box gets a legible label with voltage and ampacity. Eg 9V 200mA, 12V 500mA, etc. Each device gets a corresponding legible label with its required voltage and maximum current.
Then it's just a matter of picking out one that's adequate for the selected device, it's a many-to-many mapping with lots of valid solutions, not one-to-one.
Listing labs are a known scam. you pay them significant amounts to curb what result you want. then you change the components to cheaper quality and sell those.
The correct and effective means to have safe and correct products is to monetarily and criminally punish corps that produce faulty goods.
"Rummaging through my old parts box unearthed a power supply that was a bit more powerful than the one that came with the router, and even fit the connector." Is he describing a "wall wart" type power supply? A picture would help. A teardown would help more. There are large numbers of really crappy wall-powered DC supplies available, mostly from China.[1]
Look for a UL approval marking and a UL approval number. That indicates at least some attention to quality. UL is mostly concerned about fire safety, but their basic test for power supplies includes connecting it to a load box at the power supply's maximum rating, then running it for a while to see if it overheats. This catches power supplies with exaggerated ratings.
Avoid no-name power supplies. This is a well-known headache.
[1] https://hackaday.com/tag/wall-wart/