Performance issues are about investigating and narrowing down the chinks, before arbitrarily throwing resources, although you probably did it because of business constraints. Since this worked for 2 months post-upgrade, focus on what features/changes you have introduced since the issue started.
I have linked a post, below; use those tools to narrow down the issue. Since you see frequent GCs, then the next obvious spaces are your configuration and code (possible memory leaks). Enable auditing for your parent and key services, examine any unusual resource consumption or response times.
Like I say, there are too many cogs to investigate; so proceed with the rule of elimination.
KM