GTA 5 average fps dropped within a few weeks

They released an update and patch for that so if they updated it should have fixed it,if not unlucky them.

It may be something else as since the start of GTA V i always had really good fps but once in awhile it would stutter a bit or drop,and when i tried to play the custom radio it really went to hell and sometimes was damn near a slide show.I never did find a good answer through my searching.I forgot about Malewarebytes so i closed that out and all that went away.So maybe if you installed something like that it may be giving you an issue?

Try lower some settings and see what happens,if it was working fine before it either still issues from the update or something that changed with your pc to cause issue.