Google App Engine Instances keep quickly shutting down
My solution to this was to increase the Pending Latency time. 
If a webpage fires 3 ajax requests at once, AppEngine was launching new instances for the additional requests. After configuring the Minimum Pending Latency time - setting it to 2.5 secs, the same instance was processing all three requests and throughput was acceptable.
My project still has little load/traffic... so in addition to raising the Pending Latency, I openend an account at Pingdom and configured it to ping my Appengine project every minute.
The combination of both, makes that I have one instance that stays alive and is serving up all requests most of the time. It will scale to new instances when really necessary.
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?