Hello there guy, Big Problem

i have a 1999 E280. the problem is every time i go round a corner the steering wheel doesnt go backto its original position (it doesnt straighten up), i have to do it manually.

ive replaced the:
[ul][*]wheel bearings[*]tyres[*]brakes[*]steering rack[*]steering coupling[/ul]
after all these replacements the problem is still here, has anyone got any ideas on what it could be , Cheers