I used to have troubles getting my 400EX into neutral, It would just skip to second most of the time unless it was off.
I chalked it up to me being a noob with quads, but after I tore it down, repainted, and put it all back together, it went into neutral like butter!
My bet is the cable too.
Adjust the cable where it meets the clutch on the side of the motor so that it's just barely putting tension on arm, pulling it towards the cable/front of the bike.
Snug up the nuts, and then adjust the cable at the lever end.