Here is the deal:
Shockingly, when you use 'periodic' execution in a custom monitoring extension script, the frequency must be less than 60 seconds. This is because the agent sends data to controller every 60 seconds no matter what the aggregation type is.
From page 1508 on Pro417_AppDynamicsApplicationIntelligencePlatform_21Nov2015.pdf,
I'll try to verify this, but the above documentation appears to be clear.
Karun
APM|Monitoring|Consulting