Hello Lydia,
>> We've got frequently drop outs and low speed.
These are OSI Layer1 issues that probably can be addressed in a different way.
You can check the healthy of the dsl line with:
sh dsl interface atm0
look at the noise margin lines and at error statistics.
If noise margin is less then 10 dB you can have problems.
Changing from aal5snap to aal5mux doesn't provide better line conditions.
You have the following configured:
dsl operating-mode itu-dmt
Ask the provider if they expect this from customers.
Also ask them if they have upgraded the DSLAM to ADSL2+ if this is the case you may need to upgrade the firmware of the DSL card.
A way to update the firmware is to upgrade the IOS image for example to 12.4(20)T something
Hope to help
Giuseppe