This is going to be a long post / rant sorry!

Just in the last few months some of our customers have started having connection or speed issues.
There is no common denominator between them, they use different brands of routers, are on different ISPs and are in different areas so different exchange or cabinets.

I'll give a couple of examples.

The first one had been using a Micronet ADSL2+ router for almost a year with no problems on ADSL1.
Then after the cabinet got upgraded they were still only getting ADSL1 speeds.
I spent some time doing the usual troubleshooting but could not get the modem to get ADSL2+ speeds.
The status in the modem said that it was conencted using ADSL2+ but the sync speeds were still at ADSL1.
I swapped out the modem with a crappy D-link freebie and it worked great, with download speeds around 12-13mbps.
So I sent the modem back to the supplier to get it checked / replaced under warranty however they could not fault the unit on 2 of their internet connections but they sent a different modem to try anyway.
That replacement modem, which was a different brand, had the exact same problem at the clients site so I sent it back and the supplier replaced it with a Draytek modem.
This too had the same problem but luckily the Drayteks have lots of different firmwares available to work with different equipment in the exchange/cabinet.  So I changed the firmware and that got the Draytek modem working at ADSL2+ speeds.

The 2nd one bought a Draytek DV2710, for the 2 VPN tunnels, and that worked OK for a few weeks at the clients site, although we could not get a VPN connection from some remote sites (but that's another problem).
Then one day their internet speeds dropped to slower than dial-up.
Again we did all the usual troubleshooting but could not get the speed back.
We tried two other modems, a D-link & a Netcomm, and they both worked great.
I also tried about 9 different firmwares on the Draytek which all had the same slow speed problem.
So I sent the Draytek back to the supplier for checking / replacement and again they could not fault the unit on their 2 internet connections.  They have sent the unit back saying it's not their problem.

These are just two examples, there are more that have had similar problems.

So my question is is anybody else around the country getting problems like this or is just us up here in Whangarei / Northland?
What can we do about it and who should be footing the bill for problems like this?
The client can't be expected to as the modems are under warranty, there is obviously nothing wrong with their wiring as other modems work fine and they haven't done anything to cause the problem.
The supplier can't fault the units we send back to them so they aren't going to pay for anything.
In fact in the first example above, the supplier said they shouldn't have sent a Draytek as a replacement as they didn't realise that the original Micronet was almost a year old so we were lucky with that one.