Background
We have a third party application running which has its own 2012 R2 IIS server (2x 2.29Ghz, 8Gb, VM within VMWare environment) and also uses our 2014 SQL Server. The issue started on 22 January and prior to that the site has worked fine for 4 years.
For the last 3 weeks, seemingly intermittently the web application has become unresponsive for anything from 20 seconds to 20 minutes, however CPU/Memory/Disk access is fine. Performance Monitor shows a big spike in Active Server Pages "Requests Queued" and "Requests Executing" over the same time period. Other counters (Drive, Network, plus the SQL server access) all look normal and do not spike.
The web server has been migrated to different hosts, and even to a different data center altogether. In addition we have also spun up another 2012 R2 server and migrated the site and the problem follows.
We have 2 other instances of this product, each with a dedicated web server and neither have this issue.
Using performance monitor we can see the request queue spike by 5x and in turn drive up executing time. Is there anything in IIS we can use (knowing the start and finish time of the issue) to potentially see the trigger of this spike?
Any help, advice and pointers appreciated!