Ok, when u shorten the actuator rod, it means the wastegate opens later, and obviously - it can't open as much as the rod is shorter and it runs out of travel.

What can then happen, is that as the revs rise, the wastegate opens, but not enough to let exhaust gas go through to keep the boost at it's set level. The left over gas is forced through the turbo instead, spining it up more, and the boost rises with rpm.

It's a slow process, so u only notice it in 4th and 5th, as the other gears are over too quickly.