Update your post to include full system hardware specs and OS information.
Use Task Manager and Resource Monitor to observe system performance. Use both tools but only one tool at a time.
Just open the tool window and see what changes, if anything, when the GPU fan spins up and down.
Process Explorer (Microsoft, free) might also help find a potential culprit.
https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer
Another possibility is the overall arrangement of fans and resulting airflows. Similar to what can happen with an inappropriately sized room window air conditioner. Cycles on and off.
Too much airflow GPU cools down quickly. Fan spins down, then all again gets too hot quickly. Fan spins up again.
Etc., etc., cycling.....
Take some photographs of the case and fans., Post here via imgur (
www.imgur.com).
Show the current airflows with respect to fans (intake, exhaust).
Someone may spot something amiss.
Just my thoughts on the matter.