Can I downgrade from ADSL2 to ADSL?

[#1]

Hi,  I recently changed from ADSL to ADSL2. When on ADSL i had a very stable connection syncing at 5500 and with almost constant speeds of 4000Mbps.
Since changing to ADSL2, my connection has been terrible.  The SNR margin blips all over the place, and I drop connections every couple of days.  I didn't see any speed gain since the change and in fact have just checked to see my IP profile is set to 3000Mbps.  So now I'm much slower.
I've not changed any hardware am running in the master socket and have no internal wiring.
How can I go back to how things were?
Cheers

[#2]

What router are you using?
If the Plusnet supplied speedtouch  you can configure it to use ADSL1, ADSL2 or ADSL2+.
http://community.plus.net/library/hardware/forcing-adsl-modulation-on-thomson-585v7/
Telnet to the router and login using your normal password then type the following in blue at the command line
for ADSL1
xdsl debug multimode config=g992.1_annex_a
for ADSL2
xdsl debug multimode config=g992.3_annex_a
for ADSL2+
xdsl debug multimode config=g992.5_annex_a
To revert back to auto-negotiation
xdsl debug multimode config=t1.413issue2+g992.1_annex_a+g992.3_annex_a+g992.3_annex_m+g992.5_annex_a+g992.5_annex_m
To save setting and cause a reboot use
saveall
then
exit

[#3]

I'm not using a plusnet modem router. However, I've just changed the modem modulation type from ADSL2 plus to GDMT which I believe it was previously on. I'll see if that makes as difference and my ipprofile increases. Thanks