Rear Shocker

[Andy FZS]

I've been getting some slightly unnerving weaving from the rear of the bike once I get over about 60mph, I thought it could have been wheel badly balanced perhaps but I then noticed the preload on the rear shock was at about half way so I wound it fully up and things have improved. Is this normal? is there any thing to check over the winter or a mod / new shock? :\

Cheers Andy