Simyo Spain €5/month data (upto 500MB)

[inquisitor]

The problem surely is network congestion and even if you reached an English-speaking customer care agent, they wouldn't be able of changing anything. Also I think they are quite aware of that problem, as network operators have automatic monitoring tools for network load.
Are you using 3G or GSM right now? If you are using 3G you could try switching to GSM by changing the network mode in your phone. That helps sometimes at least here in Germany on the congested eplus network, as the GSM network is often less used for data than 3G.