You are certainly correct, the multipool (hashco.ws) is driving up the difficulty (by only by 41% at a time). Unfortunately, recently the multipool stayed with the network for 2 difficulty changes for nearly a 100% increase. Now we have some very long block times.
A change in the difficulty re-targeting method is one way to handle this. Several coins have made these kinds of changes. GoldCoin's method was updated at the end of August, which minimizes some of the damage done by multipools compared to other coins such as DGC and IFC (before they were updated). The idea behind the difficulty retarget is that the difficulty should increase in such a way as to make the coin less profitable compared to some other coin, without having a drastic increase in the difficulty that would make the network very slow after the multipool left.
The DevTeam is looking into this matter currently.
