I love linear overpull brakes, but they do take some work.
My advice...
Break down the whole brake assembly and wash out the grime, regrease all the turning points.
Reassemble on middle tension peg, only use high if you've got weak return springs and these sound new, so you shouldn't have that problem.
Loosen the adjustment screws to about mid point, and adjust the brake cable, then dog it down tightly, center the brakes with a touch to the levers, and fully squueeze down the brakes. Whatever they return to, correct half of that with the adjustment screws. When you fully squeeze then, they should come out straight.