i'm not sure i fully understand the problem (could you elaborate on the speed wobble?) but perhaps part of the problem is that you don't lean back enough or move to the back and side of the board enough. As you are starting your second seaqson i'm wondering: how is your footstrap surfing? Secondly, what conditions do you fly in? Fully overpowered required a different attitude then underpowered or powered up .. Usually when you lose controle you end up going down wind i suppose, you can compensate this by litterly pushing with your feet on the side of the board or better yet by placing the footstraps more outboard and getting in them (the rear ones might be a hassle but once you get it, you're off and i've surfed overpowered out of the rear footstraps aswell with very few problems).
Also a way of controling the speed and the sail is by rather then leaning back in your harness, is to put yourself in a more sitting-like position (show off that bum!), it'll decrease your speed but you'll feel safer and it'll permitt you to move a bit more freely untill you get that hang of your board.
When i first read your thread i was thinking of overfinning: fin = too big, but seeing as you're usuing it with a 7.8 on a 145 L board i doubt it, however, it might be one of the factors why your board is more unstable with smaller sails (my recon 5.0 - 6.7), you migth want to consider like a 48-er or so ... Overfinning can be one reason of losing controle; i once was forced to overfinn and even though i very much enjoyed the speed i got, i was not the smoothest ride i had nor the safest or most confortable.
hope i said nothing wrong, and if so, Roger will correct me, i hope ...
anyway, hope it helps Roly,
Haiko, AKA crazychemical