my 50p bet goes on fucked wishbones.... either the bushes are perished or split...(check them in a loaded state i.e on the floor via a pit..or a 4 post lift)..or the ball joints are going home.tracking will only become out of adjustment for a couple of reasons. giving kerbs some love or big pot holes..that usually wanks wheels aswell

or somethings buggered and its not right in the first place. you didnt say which ones were checked but the wishbones are a rubbish design. last less than 5 mins on a lowered car..and even less if you pedal it round corners like a go kart!, ive had ripped bushes from a couple of emergency stops (escos front brakes loved doing that to them!) and a squeaky balljoint which was super irritating. phils replaced that many pairs he wishes he got airmiles for them! pattern parts are 20 odd quid a side. 3 bolts and a bit of elbow grease and youll be sorted.
and just to check. you are testing it on a flat floor and not a road which is either tramlined or has a visible camber..they all camber to the left to allow rainwater to run off but some are worse than others....(dont laugh its been done before!)
oh yeah..if its just your steering wheel that wont sit straight check for play in the steering column U/J..never know.
Hope that helps.
Jam.