My games without vsync used to get fps rates in the hundreds. All of a sudden the highest i can get on anything is 75. Pc is 6 months old. Still somewhat new to this. Gigabyte Radeon R9 380x
There might be several reasons:
Overheating is a common issue, especially on gaming. Check with a software if PC is overheating,due to maybe dust buildup.
About the fps drop, it is possible that there's a running process/service, possibly an OEM software or malware.