Is a new Router likely to improve and help stabilize my DL Speed ?
Yes, although in a lot of cases just changing the software that runs on the router can help.
Most 'one box' home routers are made up of two distinct circuits.
1. ADSL Bridge - This is what gets plugged into the Microfilter
2. Hardware Switch - This is where you connect your network devices
Optionally there may be a WIFI circuit, but this mostly deals with encryption of the data sent over wifi and some Quality of Service.
in between the bridge and the router is a piece of software that moves traffic from the WAN (bridge) to the LAN (switch) that software can also transform that traffic by sending it to different places and also perform analytical tasks on the traffic to ensure it is risk free, i.e. firewall, web filtering, etc. the problem is that often the circuit this software runs on isn't very powerful and the faster your connection is the more data it has to process at a faster speed, this creates a bottleneck within the router because it can only process a certain amount of information at a time, the end result is that you think your broadband is slow when in reality it is that the router isn't performing in line with your requirements, this can be born from two fundamental reasons.
1. The software (firmware) is poorly coded and/or has bugs
2. The part of the router that the software runs on is underspecification for the task.
I recently changed from a Speedtouch Business Router at home to a Draytek Business router and my broadband speed increased by approx 20-25%
Hope this helps.