This is a tricky one. As the previous respondents have correctly pointed out, the rudders should be checked and the main eased off. You may be missing a rudder, it has happened before. At 120 degrees apparent you are running pretty deep downwind so the car should be at the end of traveller and the main eased somewhat. Most cats will have
weather helm with the main up running deep downwind so you should be okay - something really is wrong here.
With your comment that the previous owner never sailed the boat, and the interesting quirk that the boat steers fine in gear, the problem could be prop turbulence from a big 3 bladed prop. The Salina has the props in front of the rudder and if this is a big
generator of turbulence the rudders may have a real problem dealing with the wake. The rudders are not very big and so a large proportion of the area may be affected.
Putting the motors in forward will smooth the flow over the rudders. Putting them in neutral will cause a lot of turbulence and cause the rudders to work less efficiently.
To see if this is the cause you may have to put the boat on the beach, remove the props and replace with feathering props. This is expensive. It is cheaper to put a GoPro on a stick and take some video of the rudders underway. Lots of turbulence will show up easily. Then put in gear and see if it goes away.
Hopefully it is just an alignment problem and the rudders are fighting one another. Jump in the
water and align one rudder centrally and check that the other one is also straight fore and aft. Maybe you have a bent rudder bracket. That is what I would be hoping for.
cheers
Phil