you've been sold the wrong inverter. For a string inverter in shading like this you need either Aurora Power-One, or SMA TL or HF inverters, or similar which have a full voltage range scan function that stops it sitting on a false peak like yours is, and enables the bypass diodes to function correctly to bypass the shaded sections and allow the rest of the array to operate at near enough full capacity.
they have also probably undersized the cable, causing the inverter to raise the local grid voltage beyond the protection limits causing it to cut out - either that or you have very high grid impedance on your incoming supply / high grid voltage anyway, or maybe a lot of other solar installs on the same line.
I'll post up a graph of a system we've been monitoring that has hard shading coming across it from 5.30pm, without this function enabled it was doing as yours is, with it enabled it's reducing output by around 10-20% as the shading runs across the array.
