Instant FPS Drop In Game - Now Permanent

Can anyone help? Been playing the game today and as soon as I zoned in elsewhere my FPS dropped to 25 (always stayed at 60FPS with settings on medium/high before).

Now when I exit the game and go back in it plays on 60FPS for about a minute and then drops to 25 whether I have the settings high or all low.

I do not know what else to do, I have repaired the game from launcher, ensured I have no programmes/processes running in the background and it will not fix it.

This was an instant, unexplained drop and it’s made the game unplayable.

Thanks in advance!

Temperatures?
Addons?