This Washington Post coverage of the Peruvian Election provides a very good summary of what’s going on. One excerpt:
Can Peruvian polls be trusted?
No. Peruvian polls are notoriously unreliable. The most solid prediction is that there will be a second round and Humala will be in it. The volatility of the polls rests on the fact that many Peruvians do not make up their minds until a few days before the election and that polls rarely reach the 20 percent of voters in the poorest, most remote areas of the country. (Because isolated, low income voters are most likely to vote for Humala, this may mean he is even further ahead than polls suggest).
Also it seems, that Flores is currently in the lead again according to the polls. I have read some very critical articles against Humala in the newspapers (though I only read the first pages at the kiosk). So the following interpretation seems plausible:
"Humala's support has fallen because of a barrage of attacks against him these last few days. ... That has generated a fear of voting for the unknown," said CPI's director, Manuel Saavedra.
But again: polls are unreliable.