What oil are you running? I have a stumble and the recommendation was to increase SU oil viscosity with a relatively stock L28 engine now. I haven't tried it yet and I see you've increased the spring tension already. Wonder if some experimentation may not be in order.
I had an L28 with a 530 lift, 306 duration Isky cam, bored, ported, etc., with stock SU's and it ran wonderful. I swapped over from OER 45's as I didn't want the hassle at the time. Wish I kept that car.