What is the cause of this problem on my 1998 Ford Escort Cabrio?

Right, I brought this car without test driving it (stupid I know) but anyway, the only problem I'm having is when the car is at steady speed the steering wheel sits to the right, when I put the clutch down to change gear, the steering wheel and car flicks to the left slightly, when its back in a gear the steering wheel is sitting on the right again after another flick to the right! This is extreamly dangerous when this happens at 70mph as these ''flicks'' I'm getting get stronger! It does the same from acceleration and deceleration!
So thats the problem.
I've taken it to a garage where I normally have all my MOT's done and work etc. and he told me that the wishbones looked fine and that the rubber bushes were also fine so he couldn't see the problem, then he said it may be the steering rack and something not positioned correctly, although I fail to see why that would cause all these very wobbly steering problems I'm having!
Anyway, just wondering if anyone else with an escort has experienced these problems and has an answer for it, because I could either end up having both sides wishbones replaced and it not making any difference after spending about £100, or, having something done with the steering rack costing around £200/£300 and that not making any difference! Help!! pleaseee!? :D
Thank youu! :)