So I put it on a Buck converter and started it out at 1 volt... it ran continuously ... but when I bring it to about 2.5V, it stops. But under 2.5, it runs. It's rated for 12 volts.
That looks like a standard "280" size motor, more or less, but with sealed metal back instead of usual plastic cap. Of course, this being from Amazon, there's very little technical information, let alone a proper product datasheet. (I love how they give a vague RPM and rated voltage range. What about stall current? Stall torque? Maximum efficiency at ___ RPM?? A datasheet could answer all of that. On that note, I really don't understand why people buy so much junk from Amazon. It's the worst place when it comes to product info. Even the most terribly translated eBay listings I've seen usually provide better product specs. And Amazon prices are usually pretty ripoff too. $7 for that motor? Hope you at least got one with carbon brushes. But truthfully, I really wish Amazon would stop selling electrical components. /rant.)
Anyways, is the motor about 30 mm long and 22-23 mm in diameter?
If so, that probably is a 280 size motor. If that's the case, I suspect it may be stopping due to worn/bad sleeve bearing(s). Was this motor brand new when you got it or was it used for a while? Being motor with vibration cam makes it even more susceptible to bearing wear.
When I was a kid, I had at least a dozen of toy RC cars end up with seized drive motors like that after many years of use. The way to fix a motor like that is to disassemble it and clean/resurface the sleeve bearings - something probably not worth doing on a cheap/small motor like that, especially if it has a metal back cover (harder to remove.) But it is doable and similar to fixing sleeve bearing computer fans.
Of course, before you conclude anything, how about first try connecting the motor to a known good DC power source to see if it stops or not. I suggest using a standard 5V power adapter or something similar. If it doesn't stop, then whatever circuit you had this in may not be functioning or driving the motor correctly. Otherwise, it's likely worn/bad bearing, as I mentioned above.
* OR *
If the motor stops randomly after running for a while, it is also possible that it may have bad/misaligned/worn brushes. It's rare, but it can happen sometimes on these small cheap motors.
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