Question about retarget system bitcoin, add hardcodet block to exist blockchain
if (nActualTimespan &lt; params.nPowTargetTimespan/4)
        nActualTimespan = params.nPowTargetTimespan/4;
    if (nActualTimespan &gt; params.nPowTargetTimespan*2)
        nActualTimespan = params.nPowTargetTimespan*2;


This does the opposite of what you want. It increases the difficulty quickly, but decreases the difficulty slowly.

Here's what you meant to do:

    if (nActualTimespan &lt; params.nPowTargetTimespan/2)
        nActualTimespan = params.nPowTargetTimespan/2;
    if (nActualTimespan &gt; params.nPowTargetTimespan*4)
        nActualTimespan = params.nPowTargetTimespan*4;