One of the problems with the PIIGS is widespread official corruption. The core countries have low rates of official corruption typical in the industrialized, Western world and Asian countries like Japan and South Korea.
Greece, Spain, Italy and Portugal all have problems with official corruption. When citizens notice that officials are not paying taxes, have secret bank accounts and openly accept bribes to cut through government red tape, people follow the example of their leaders and evade taxes, steal electricity and pay bribes to avoid following the law.
Corruption in these countries is standing in the way of economic reform. If I had to guess, I would say that Spain has the second worst corruption problem behind Greece. Check out the link above for an interactive map of all the current cases in Spain.