I think the problem is not in Slush's pool but in that the big pools like Antpool are cheating somehow.

Look the whole network should generate 144 blocks roughly in 24 hrs. The power of whole network is around 700 PH. The power of say Ant pooll is ~ 160 PH. So it should generate around 32 - 33 blocks per 24 hrs but it generates 56 - 58 the last week.

So Slush and other relatively fair  pools lack blocks