Sitecore recommends a minimum of 16GB of RAM when setting up the infrastructure. Please see the Installation Guide from Sitecore [here][1].

The issue that you may have experienced also is maybe due to the fact that there were an increase in the number of queries to the Solr server. 

Maybe try also to see if you have any query causing performance issue.

**UPDATE**

There are different profiling tools that you can used. Paid and free versions.

1. DynaTrace - Paid
2. YourKit - Has trial
3. Stackift Prefix - Both free and Paid version
4. [SPM][2] - This service will expose request rates, latency, all Solr cache info, JVM memory, GC, CPU, load, disk and network IO, etc. It has a free trial


  [1]: https://dev.sitecore.net/~/media/Downloads/Sitecore_Experience_Platform/8_0/Sitecore_Experience_Platform_8_0/Non-secure/Installation-Guide-SC80-A4.ashx
  [2]: https://sematext.com/spm/