It sounds like you've done all the basic things that might alleviate your problem, and I'd suggest that your diagnosis of the bandwidth being throttled to be the most likely. If you live in a village maybe when the kids get home from school and play online games there will be a drop off and the same for when adults finish work, but if there has been an uptake of the new TV service and lovely Movistar haven't upgrade their capacity locally, then things will start to fall apart quickly. Have you considered satellite internet ? I was looking at a UK based provider a while back who also covered Spain and some other european markets. Their prices weren't so unreasonable and the purported speeds both up and down fast.
SHJ