There is your problem, if it loads too easy, then the sabot will NOT engage the rifling properly and you will get blowby in your sabot. You need a thicker sabot--an MMP12. You need to have a "Goldilocks Sabot" now what is a Goldilocks sabot? It is "just right". Remember Goldilocks sat on the beds and the first was too hard, the 2nd was too soft but the third was 'just right'. Sabots are the same way.
Some are too hard going down, you have to pound them with a hammer. Some are too easy, like yours, but some are "just right". Just right is TIGHT, but not pound it down the barrel tight. IN ALL CASES, loading with a swabbed barrel is necessary, since a "just right" sabot will usually NOT go down a dirty barrel. So get some MMP12's and get some Crushed Rib sabots, both can be bought here:

http://www.midsouthshooterssupply.com/item.asp?sku=0021150045HPH

http://www.midsouthshooterssupply.com/item.asp?sku=00072H15045BR

Now if the SpitFire has a boat tail design, I think the TMZ does, then you have to place a #8 washer in the sabot so the bullet can sit on it, the #8 washer keeps the sabot tight to the rifling.The TMZ has a special designed sabot. Also try the Knight Red Hot, whichis specially made for Knight Rifles by Barnes, they SHOULD have atighter fit.

Try these procedures and let us know how you do.

Best wishes.

Chap Gleason