dead cheap 20Mb/ADSL … at what cost

Over 6 months ago I got a 20Mbit/s ADSL2+ connection with here in Spain. Of course you never get 20Mbit/s with those connection but I got a nice 15Mbit/s. Perfect for getting something immediatly, watching HQ streams without bothering about somebody else using the net in the house, etc… All in all, I was really pleased about it…
…until a month or two ago, where starting from 6pm the speed drops down to 512kbit/s (if I’m lucky) with very high ping. Of course that is the time where I want to watch some streams to chill out, maybe do some work from home… GRRR
After further investigation, it’s caused by my DSLAM server being saturated at those times. And by looking on spanish user forums, I’m far from being the only one with a saturated DSLAM.
So my question is : How long are those ISPs going to carry on with all that saturation without doing anything ? Switching ISP is not an option, since it will be using the same DSLAMs except if I switch to Telefonica (state operator) which has its own DSLAMs… at 150euros a month for the same connection (yes, that’s 6 times more than what I’m paying).
Calling doesn’t seem to change anything (except for being charged 30mins of wait on a premium number). Was there anybody in the same situation ? What did you do to pressure the ISPs to add DSLAMs in saturated regions ?