Strange connection - IP Address changing?

[#1]

Hi, I've allways had issues with the IP on my connection switching sometimes every week but it's not constant but more recently it has raised my attention since my line profile has been set to 5meg from a usual 11meg.
This started when my line had apparently been dropping out which I haddn't even noticed.
About a week ago I started getting a whole day of random disconnections and when the line came back it was ranging at 0.32meg and 1.80meg.
When I finally got my connection stable again my line had be capped at 5meg when I have had what I believed to be "stable" internet for around 5 months of up to 11meg, Though I usually only got around 9.83 to 10 meg connection.
Could this be to do with a phone line extension cable I have coming from the line socket which adapts to an ADSL filter which splits the phone from the router?
How oftern if ever should my IP change without me actually manually rebooting my modem? Is this something Plusnet does or has there allways been something wrong?
I have done a series of tests over the past few months and my ip can change up to around 6 times a month.
Though I have had up to 11meg before I have allways been able to download and use the internet like normal and never noticed any obvious disconnections as this happend rarely.
Generally I would like to know if there is anything I can do to maintain a "stable" connection even with a greater speed of connection? Would using a long high-speed ADSL cable in replacement of my phone line extension work?
Here is the line information from my modem/router
ADSL Link              Downstream    Upstream
----------------------------------------------------------
Connection Speed     5448 Kbps    446 Kbps
Line Attenuation       31.6 dB            19.0 dB
Noise Margin       16.8 dB            20.9 dB
Here is the results of my speedtests ran over this year.

in late feburary I got 12meg around the time I believe I had interleaving enabled.
since then I had an average speed of 9.8meg.

[#2]

The easy part first - the IP address may change - it doesn't have to but usually does - when the router loses it's PPP session.
This doesn't necessarily mean a loss of sync.
For example it can lose it when Plusnet change their mirror of the IP profile.
Regarding the extension lead to the router/phone.
This can be a major factor in speed problems since the usual extension lead sold for phone use is terrible when used for broadband.
How long is the extension lead
The normal best practice is to locate the router at the master socket and run a good quality cable from it to the computer assuming that you don't use wireless.