GTA V Rubberbanding and lag galore!

[#1]

I am feeling completely confused and extremely frustrated with my delayed gaming experince... I am able to play online with three of my friends, which seems to be a blessing as we can perform heists and general muck about. However, when we join a busy server, my connection has a fit. It's almost like my connection hits a cap, and I start to skip all over the place. Even my voice on Teamspeak becomes ineligible. I sound robotic and no one can understand a word I am saying. I can't shoot anyone unless they are standing still, and I get obliterated by everyone else. Eventually I get dropped from the map as I see everyone leaving the game. As soon as this happens, everything is back to normal. At first I thought it was an issue with Rockstar, but I have learnt that it's all peer to peer, plus my brother also has the same connection speed as mine (but a different isp as he changed to talktalk) but he doesn't get any distorted voice com issues, and he's able to play on a busy server without dropping from everyone else.
I have tried many things which I'll list below
Updated the firmware on my router,
Checked ip settings,
Enabled/disabled Upnp,
Enabled/disabled DMZ
Checked drivers are all updated,
Checked DxDiag
Adjusted MTU to 1473 & 1450
And a few other steps that I'm currently too tired to remember.
The only conclusion is that it could be something wrong with my connection setup. I'm a little rusty with my network terms, but as far as I understand (After being sent the incorrect router twice as I originally wanted a router/modem)
From my understanding, I have my connection setup with my default supplied modem router (provided by plusnet) working as a dedicated modem, and my router (RT-AC66U) which handles any wireless connections. My PC is wired to my router, so I can rule out any wireless issues. My max line speed is roughly 8Mbps, so I'm using (I think this is correct) An ADSL modem that's configured as a bridge, and my router is connected to that.
If anyone has any suggestions, or ideas to try, I would LOVE to hear them!  
Spec:
i7 3.5GHz,
Asus Z87-Pro,
16GB RAM,
Nvidia GTX 970.
WIndows 8.1 64bit

[#2]

Only issues ive had is with the beta driver i downloaded, it was supposed to optimize the game to run slightly better but it infact made my game gfx poor so i rolled back to the latest stable patch and all is well.

[#3]

Not much you can do if hosting is peer to peer.  You are dependant on the host having a good enough pc to run the game and still have enough cpu grunt to handle dealing with the other players games.  Plus a decent connection, with good upload speed that isn't shared eg in a family and all are online trying to what the latest cat video.

[#4]

Thanks for replying. I have a suspicious it's a combination of two things. I have noticed that GTA V is very demanding on the CPU. And I also think it's my line. I have noticed when I alt tab out and I type into my browser, the text input is entered slowly after I typed it.  I could try and dabble into the realms of overclocking. If I can squeeze another 500Hz out of the CPU, I think it might help.
St3, I have also downloaded something that was supposed to optimize my gaming experience with GTA, but I will try rolling back to the previous drivers to see if that helps. Thanks for the idea.
And I totally get what you're saying Gus. My family have discovered the joys of youtube... suffice to say, it has an impact on the games I play online. There was rumor that we might be getting an upgrade to our area (ADSL2+ possibly) but I'm not sure if it would resolve all my problems. Faster download and upload rate would definitely be a welcome addition... and it will probably be just when I move house... 
Oh, I just remembered I have a feature called Quality of Service on my router. I'll give that a try as well...
TTFN 

[#5]

FIXED! 
Just to update, my plusnet modem router was the problem. Just replaced it with a different ADSL2+ modem (which I would need eventually) and it all works perfectly. No lag or stutter. Not sure why, but maybe it was starting to fail as I had it for a while.
Just need to work on my aim...

[#6]

Good news matey Now get some heistz completed

[#7]

I've gotten bored of the heists, they were fun to begin with, but, doing them over and over again gets tedious, especially the last one trying to escape from robbing the PacStan bank...

[#8]

haven't played the heists on PC yet but have done on Xbox's....
Amazing how fast those police can go... and accelerate up your .... when your on the bike.. 

[#9]

The crew I'm with decided to ditch the bikes and take whatever cars we could find instead, the bikes are fun with that electric boost feature, but in the heist, they're slow as hell!! 
Still, that van that the bank heist unlocks is fun, sounds like a diesel ford transit, just without the 150 decibel noise inside the cab....

[#10]

Yeah I do exactly that too... raced up their in a Conquete the other day... got to the boat quiet quick