First of all you have to be sure that it's a fin and not a sail problem that makes you spin out. I (95 kgs) have an iS 122 and I don't have any problems with my Drake 46 with a 8.5 sail - except that it actually feels a little too big - and I can use the Drake 40 comfortably (and faster) with the 8.5 when the wind is higher and steady.
How is your sail trimmed? Do you use enough downhaul (lots of surfere don't), and do ylu have an adjustable outhaul? What about foot strap position? Are you in balance on the board?