FPS stutter rdr2

In rdr2 my fps starts off fine (70-110) every minute or so however to goes straight down to 10-30 and then back up to 70-110. It does this every so often and it makes the game unplayable. I’ve tried everything yet nothing works. When I have an external monitor plugged in however, the game runs fine on max graphics. Without the monitor with lowest graphics even, the game stutters ever so often.Is there any way to fix this?