Is it possible via the Command Line interface on a 2820n to set upper or lower speed limits that the router is allowed to sync at. I seem to be having a see-saw type problem where the router tries to sync too fast, does quite cut it and looses sync, then backs off way to far and gets a slow speed. I would like to be able to set a value somewhere in the middle at the expense of speed in favour of stability.

I think it's generally the equipment at the exchange end that that sets the speed.
You could try getting your ISP to increase the SNR (or do it yourself if you have the ability)

The 2800 had this facility but 2820 chipset does not.