Google App Engine Instances keep quickly shutting down
I only started having this type of issue on Monday February 4 around 10 pm EST, and is continuing until now. I first started noticing then that instances kept firing up and shutting down, and latency increased dramatically. It seemed that the instance scheduler was turning off idle instances too rapidly, and causing subsequent thrashing.
I set minimum idle instances to 1 to stabilize latency, which worked. However, there is still thrashing of new instances. I tried the recommendations in this thread to only set minimum pending latency, but that does not help. Ultimately, idle instances are being turned off too quickly. Then when they're needed, the latency shoots up while trying to fire up new instances.
I'm not sure why you saw this a couple weeks ago, and it only started for me a couple days ago. Maybe they phased in their new instance scheduler to customers gradually? Are you not still seeing instances shutting down quickly?
So I've been using app engine for quite some time now with no issues. I'm aware that if the app hasn't been hit by a visitor for a while then the instance will shut down, and the first visitor to hit the site will have a few second delay while a new instance fires up.
However, recently it seems that the instances only stay alive for a very short period of time (sometimes less than a minute), and if I have 1 instance already up and running, and I refresh an app webpage, it still fires up another instance (and the page it starts is minimal homepage HTML, shouldn't require much CPU/memory). Looking at my logs its constantly starting up new instances, which was never the case previously.
Any tips on what I should be looking at, or any ideas of why this is happening?
Also, I'm using Python 2.7, threadsafe, python_precompiled, warmup inbound services, NDB.
Update:
So I changed my app to have at least 1 idle instance, hoping that this would solve the problem, but it is still firing up new instances even though one resident instance is already running. So when there is just the 1 resident instance (and I'm not getting any traffic except me), and I go to another page on my app, it is still starting up a new instance. 
Additionally, I changed the Pending Latency to 1.5s as koma pointed out, but that doesn't seem to be helping. 
The memory usage of the instances is always around 53MB, which is surprising when the pages being called aren't doing much. I'm using the F1 Frontend Instance Class and that has a limit of 128, but either way 53MB seems high for what it should be doing. Is that an acceptable size when it first starts up?
Update 2: I just noticed in the dashboard that in the last 14 hours, the request /_ah/warmup responded with 24 404 errors. Could this be related? Why would they be responding with a 404 response status?
Main question: Why would it constantly be starting up new instances (even with no traffic)? Especially where there are already existing instances, and why do they shut down so quickly?