what would cause...

[northernmunkeh]

at about 68 mph to 80mph passenger seat wobbles quite violently, can feel it through headrest. Just had 4 new PS3's fitted and balanced. Problem existed before and after...

can't seem to keep the car straight over any road with the slightest bumps, the ride seems to absorb the bump well enough but the rebound causes the car to go anywhere but straight.

and with the above problem it just seems impossible to actually drive straight. I'm having to make minor adjustments all the time and even then it doesn't feel like its consistent with the inputs on the steering wheel.. seems to just wander around on the road.

I'm thinking it might be the steering rack, or knackered suspension components but dont really know where to start 

[neg]

wheel alighnment maybe?  Worth getting it checked, most places dont charge if its ok so nothing to loose.

The rear of the Mk5 can be altered too so you need to find a 4 wheel place. 

[DanGB]

Could be the Toe

[stealthwolf]

tracking

[Andy]

tracking woudn't cause the car to wobble-when you had your tyres fitted was any of them slightly buckled

[john_o]

ok,

when did it first start happening ? has it been sudden or gradual?
any large potholes or banging the kerb?

could be

bent wheel
tyres should be fitted with the 'dot' opposite the tyre valve , ensure also all balance weights are still on..
any area that has required a huge amount of balance weights??

any unusual tyre wear on the set that came off?
you could jack the car up and spin a wheel and check to see if  its true (inside and outside edge)

to be honest is it existed before and after , its either the wheels or suspension components.
maybe swap the front wheels onto the back to see if it still happens.

are your wheel bolts tight?

if you swap wheels around and it still occurs you need to get the geometry  checked, anything bad will become apparent very quickly etc track rod end/bent lower arm etc

let us know how you get on 
try swapping the wheels front to back first