Chuck another couple of fins in the front. That'll fix the problem.

I've got a fanatic Newwave Twin 84ltr - I found the same thing with the stock fins. I was expecting it to be loose to a certain level being a twin but the board was really really loose. Got myself a set of the Maui Ultra Fins X-Twin 16.5cm and love them:
http://www.mauiultrafins.com/MUF7_english/Multi_Fins.html
I'm only an intermediate wave sailor, but for my level of carving etc i find them great. Very easy to get back up wind after a wave as well.

Try moving the fins right back as far as they will go and mast track about 1 cm forward from centre.

I've got a pair of Maui Ultra Fins X-twin 17.5cm on my Evo 86l. Set right at the back of the slotboxes. They work a treat for me (I weight 93kg) in cross-on conditions small-wave prevalent in Perth. Not tried them in decent size waves yet though.

How much do you weigh?
Lighter guys often find the Maui ultras a bit harsh and unforgiving.
The k4s offer you a lot more grip and don't break away and slide out like g10 fins. They are also wayy cheaper.

I had my fins too far forward on the twin and had the same problem. Put them back to the middle of the box and it was much better.