I'd suggest giving Claro a go. A friend said they guarantee a minimum internet speed and there's a good choice of packages. As far as I can see, they're cheaper than Movistar as well. Unfortunately, I can't vouch personally for Claro, because they said that my building has a contract with Movistar and no one else can use their cable.
I've been with Movistar for a few years now and I've never had decent internet speeds, despite upgrading from 2mbps to 4mbsp and I'm now paying for 6mbps but rarely get more than 250kpbs. The other day I got a call from Movistar asking if I wanted to upgrade my service again. I said, what's the point? Do you want me pay for 10mbs and then you give me a speed of 300kbps?
One thing I'd also suggest is taking out your contract through the internet as there are some good discounts on their webpage at the moment. They'll tie you into a year's contract, but that's standard and they'll give you the first three months at a reduced rate. By the way, if at the end of the year, you think the service is no better than Movistar, you can always go back and take advantage of their internet deals, which also offer three months for reduced rate.
Hope that helps.