One thing to keep in mind is that actual twist may deviate from nominal twist. Just because it says it is 1:8.6 doesn't mean that your specific barrel isn't really a 1:9.5.
The other thing to remember is that its really about bullet length and RPM, so a lower velocity cartridge might not stabilize a specific bullet but a faster one might.
Are you sure its a stability issue and not simple inaccuracy? Some barrels don't like some bullets, stability be damned. Those very same bullets might work fine 200 fps slower in a slower twist in another gun, they just aren't compatible with yours. Every component you're working with might be really very accurate ... but not when used together in your specific gun.
Tom
It could easily be inaccuracy.
