If you were on to him for 20 minutes I assume the eircom helpline guy did go through some possibilities with you. When I got eircom broadband about six months ago I found I was getting very low download speeds (although higher than 121 Kbps) so I called their helpline and the guy on the other end spent at least 20-30 minutes getting me to disconnect and reconnect various bits while he kept monitoring the speeds.
Turned out with me that I had left one of the phone connections in the house without a filter. I thought it wouldn't matter if I wasn't using the phone on that particular connection. I could not have been more wrong. All of the connected equipment in the house needed a filter and once I did that the speeds improved greatly. Since then I don't believe I've had any problem of substance at all.
Maybe I got lucky and hit upon an eircom guy who actually cared enough to try to get it to work and he wasn't going to give up. He was an overseas guy, Indian I think.
They also replaced two faulty modems for me in the first week in quick succession, without any hassle or delay. So my eircom experience has been pretty positive to be honest.
By the way, I have 2 Mb broadband and I usually get downoad speeds above 1 Mb even with the contention ratios they have. And when I called them the guy on the phone said I was quite right to call and make sure something was done. He said he reckons loads of people don't realise they are getting lousy speeds. They think it's just normal 'cos they hear others complaining so they never sort it out. So I would say keep at it 'til the speed improves.
Sorry if I don't have an answer to your specific problem (unless it's an unfiltered line
) but I hope my experience tells you it's right to pursue this.
Cheers.