Steering dives over 40mph, producing a speed wobble!

[GT Robs i]

Any help would be great.
Mk4 125hp gti, changed the wishbones, track rods, and ends, ball joints and pads and discs on the front,both sides, also had new 225/45/17 and had the tracking done, and checked.
Driving over 40mph usually on a duel carridgway as you feed a little steering wheel to pull back in the car will dive to that side as if the front wheel has folded in!
There is no exsessive play in the steering wheel, and the power steering doesnt seem to be leaking or making nasty/expensive noise.
Under 40mph its not there or any other problem with the steering.
I don't want to change the rack as it looks like a b!tch to do, and can't see any other problems.
Any advise or ideas as to pin down whats wrong would be wonderful.......

[SI74]

Take it they balanced the wheels when doing the tyres?

If you jack the car up on one side is there any play in the wheel if you push/pull at the top and bottom?

[GT Robs i]

Yep balanced, every thing, I checked every thing for tightness yesterday and had another look at the rack, Theres no play in the steering, or leaks etc on the rack.
Im going to try putting the rear wheels on the front as the only thing different is the tyres, they are cheapies!
Cant find any movement in the strurts, and it only happens over 40, so Im thinking wheels or tyres.

[SI74]

All wheel bolts tight as well.

[GT Robs i]

yep, f###ed about some more today, changed the power steering fluid and I think its the pump, The lack of answers on here made me go through it a bit more logically, The steering continues to steer if you apply slight direction, it only happens over 40 and when the whole thing is warm, so Im "hoping" its a weak pump thats slow to build pressure. Either way changing the pump is easier and cheaper than the rack, but knowing my luck/car it will need doing.
After changing the fluid today the pump now makes an I'm tired and old noise. Fingers Crossed