340 idles at 1500 won't stay running in gear

i checked the intake manifold bolts they where tight, pulled the needle seat and found whitish crust around the o ring, after cleaning it seen the o ring was turning white and was like dry rotted, it was still one piece but you could see very small cracks going around it. also the o ring itself was able to slide around, i dont know if it applies but i seen a video on youtube where someone was saying it shouldnt do that

going to get a new one and see what happens