No, I'm a BGA repair tech that has had to deal with the bad RAM chips on that model of phone, among others.

The RAM will appear to pass hardware checking in our testers, but you hit it hard and suddenly the frequency becomes unstable, often dropping down to DDR1 speeds. Plenty of room for error, there.

Ugh, I'd suspected something like that because it behaves very badly sometimes, but it's intermittent. Is there any recommended test for this? Or is it too ephemeral to test for?

You need a dedicated BGA removal/reflow tool (like a Metcal) a compatible RAM board to mount the removed BGA upon for insertion into a RAM tester, then you need a hardware RAM tester like RAMCHECK LX.

That's an expensive set of kit.