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From what I understood, you are facing intermitent issues while working with ACI instances.
There are some situations that you may are experiencing limits on the requests, or limitation on Azure container services.
Please check the documentation below to better understand limits.
Some other interesting docs - https://learn.microsoft.com/en-us/azure/reliability/reliability-containers
Also please using Azure Log Analytics, to monitor your environement to identiy possible issues.
https://learn.microsoft.com/en-us/azure/container-instances/container-instances-log-analytics
Regarding availability, if you are looking for reliability and resilience when delivering your service. I would like to recommend to use AKS cluster (kubernetes), as ACI instances don't garantee high SLA and the infrastructure can change as needed, following Microsoft monitoring systems.
WORKAROUND
In the other hand, if you identify any issues with container instance being not stable, the recommendation is to STOP/START the ACI group, this will change the instances running in a specific cluster, moving the instances to another CLuster that are more stable and healty from monitoring perspective.
If the issue still happening, please reach out to our MS support team, opening a ticket and we will work on it.
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Thank You.
Lisboa