Please excuse the previous non reply, missed this one mainly due to too much travel etc in that time.
As long as the fin is not damaged (or defective) then I agree and add to what Peter has commented that some of the factors may be coming from outside the fin itself...
A key issue is that (if the spin is happening ONLY on hi speed downwind runs when well powered) then can be typical of draft/trim control issue caused by a much higher wind loading on the sail (extra wind loading with hi wind plus extra apparent wind from extra board speed) so you need to make sure that your rig is well tuned (correct downhaul for hi speed - to lock the draft well down and fwd in the sail, as well as release the top to lower drag and avoid "lifting" the rider over chop - as compared to mid or low speed beam reaching ). Batten tension should be good (not too much, but definitely not too little for top end) - and while you would typically run a loose (less positive) outhaul for downwind speed, too much negative can/will cause even the best sails to lose draft stabilty - leading towards spinout. Other rig factors that you should check are that your harness lines are long enough (for a good stance at this angle of sailing, usually longer than beam or upwind angles) and (important!) the position of the lines allows a balanced loading onto the rig(boom) at that downwind sailing angle and/or at higher speeds. Boom height too (as Peter wrote) all as part of the "balance" equation. Sounds like a small point here, but get it wrong and you will not get the ideal trim, and one of the usual factors is an overload of power towards the back of rig, compensated by extra sheeting effort thru back hand which in turn requires more back foot pressure (to counterbalance the extra back hand pull) BUT IMBALANCE your OVERALL trim..
Regarding the strap settings, one thing you may like to try is to move the REAR strap forward to the front hole setting ; this has the effect of being able to "calm" the board at hi speed across chop, and (especially for medium height or weight riders) may offer some advantage in these conditions. Forward setting is not always slow...
Regarding technique (which in turn can be influenced by correct rig trim/tune for you in these conditions) one thing to focus on is to really (try to ) stay locked on the power when you are really blasting (in that mode). If (for rig trim reasons - or because it seems to crazy wild) you start to back off and open the rig at full "on the limit", you will release the mast foot drive/pressure and the whole board trim is gone in an instant. If you are already on the limit, then what happens next might be a bit messy, and often equated to a hi speed spinout - or an unscheduled swim.
You faster you go on the rough downwind and the wilder it gets , the more really have to have the confidence to stay fully on the power (which is not just the back hand sheeting in, but the critical drive thru the rig/mastfoot). At first, it might seem like suicide - and often you can nearly prove this as you beack off and it comes true !
But if you can develop trim and then technique (confidence) to keep the (balanced) power really on (and i mean REALLY ON) downwind thru the rough stuff, the results can be significantly better than you would normally thing "possible"....
(anyone been or seen some of the midsize 2008 iS fin / base "destruction" testing we've done sailing 34cm samples in iS101/6.7m will know what I mean - and yes we did a lot of testing in this mode. And yes we broke some. And yes the history of production fins is not fantastic (c/w top end custom) but the 2008 Drakes that made it to production are pretty good - and you should - with everything else being OK be able to get some really good performance from the stock Drake in that mode)
If you want to do quick check on the actual fin, try and swap/borrow an alternate fin (as close to yours as possible) - back to back test (changing nothing else !) in your conditions. You may also find that switching to a slightly more raked fin in these (high/er speed downwind choppy) conditions makes your life (or confidence) a little easier - but you should not need to switch to a bigger fin in these powered 6.7m conditions to reduce spin out.
For use with the 7.8m (not really the problem as questioned or discussed here) then typically the best fin combo will be something in the 42/44 range while the 7.8 is in or near it's sweetspot. Samller fins could be used in fast, overpower 7.8 mode or if you were trying for GPSSS "top speed" (BROAD DOWNWIND) in lighter (7.8) conditions.
Please try all these factors and let us know how you go...
Cheers ~ Ian