Whatever's causing it, easy enough to rule out the RARB just by removing the drop links. It's a quick check...

top tip!

My car is fully superpro polybushed, complete r32 front end and 25mm eibach roll bar to match front end and jom plus coilovers

hmm, well what about your tyres also have you had a proper four wheel alignment done?

Quick comment on ARBs (general note but seems relevant).

They are a form of torsion bar, designed to resist differential movement of the wheels on an axle line. This means that, if you hit a bump or pot with one wheel, the body tends to be rolled more by the imperfection the stiffer the ARB is.

Exactly this

The ARB resists the twist beam operation

There are slightly better ways to stiffen the rear axle, allowing for more vertical independence between the wheels

The OP has managed somehow to get the frequency at which the yaw becomes apparent to an entertaining speed

OP have you tried driving through it ?

Trust me the last thing you want to do it try and drive through it

It is quite a serious sway!

Needed new pants on a few occasions

Out of interest have you tried playing with your tyre pressures on the rear? Mine is more stable with the rear ones at 2.4 bar

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OP, how did this turn out for you, did you get to the bottom of the problem? I have the exact same thing, saw the steering wheel back and forth at about 60 and the car continues to oscillate side to side on it's own. Kind of like I have a huge half full tank of water in the back, you set it sloshing side to side, let go of the wheel and the steering wheel still turns left and right!

Mine was in a straight line mainly at motorway speeds,

Its alot better with rear end lifted slightly and ive finally killed the dunlops tyres on mine so new tyres after winter and see if things change.

Well i took the neuspeed rarb off before i sold it and problem solved

So dont run a 28mm rarb without uprating the front!

Cornering speed may increase but straight line stability sucks

OP, how did this turn out for you, did you get to the bottom of the problem? I have the exact same thing, saw the steering wheel back and forth at about 60 and the car continues to oscillate side to side on it's own. Kind of like I have a huge half full tank of water in the back, you set it sloshing side to side, let go of the wheel and the steering wheel still turns left and right!

So check if you have a RARB. If you do, back it off, remove it, or uprate the FARB.

Well i took the neuspeed rarb off before i sold it and problem solved

So dont run a 28mm rarb without uprating the front!

Cornering speed may increase but straight line stability sucks

 

without sounding like a stuck record we did say this before fitting.

 

28mm rear with a stock front bar is NOT a good match and will cause issues such as you found.

 

glad you got it sorted btw!

Good to know!!!! Happy you sorted out mate!!!

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Interesting this. I've had it happen a couple of times, both times I has a fair bit of weight in the boot. Coming off a roundabout and joining a motorway slip road. Both times I wasnt hanging around but certainly not pushing it. Bloody scarey as there was no warning. Seems to be ok since I got better tyres.  

Crap rear tyres are a recipe for disaster.

 

As I painfully found out recently, despite knowing this already AND driving like a saint who'd been spiked with proper hash brownies BECAUSE of the crap rears...

I often wonder if a better rear spoiler would improve things as it when i put a impreza low level wrx spoiler on my old volvo s40 just felt alot more planted

Mostly in a straight line though, and not at most legal speeds.