faults like this can be difficult to diagnose scottie.
it`s going to have to be a process of elimination, does it still happen if one motor disconected,are the prop shafts flexing under load is anything binding on the shafts os is there a bent shaft ?.
i`m presuming you`ve already been down this road, but it`s maybe worth starting from scratch again and checking from a fresh point of view.
i have heard of people having problems with the direction of rotation of the screws.a model i have , had a similiar problem ( but it`s single screw) that it constantly pulled one way, when teh builder replaced the prop ( i cant remember if it was from a left hand turn to a right hand turn or vice versa) this apparently alleviated the problem. maybe a slight twist was built into the hull ( it`s plank on frame) but no obvious reason could be found. just some thoughts on where to start scottie .
