UCF20 Steering Vibration

[Makaveli]

Around 80-100km/h occasionally I have been getting vibration in the steering wheel when driving on the motorway without doing any braking. I have noticed this after the steering rack and pump have been rebuilt. I have been told it might be to do with wheel balance, so I ended up getting 4 new tyres, wheel alignment and wheel balancing.

It's a 1996 celsior, running 235/40 R18 at the front and 265/35 R18 at the back.

Anyone came across this before? Does anyone know what it can be?

Thanks