could be as simple as a bulge in a tire. or it may need an alignment, check your tires, are the wearing out on one side? Is there a bulge in any of them, If the answer is yes to any of those, id recomend new tires and an alignment, if your tires are fine id take the car in for a wheel balance and an alignment, generally thats all that causes the wobble at higher speeds...
For a test, get going down the road say 35-40 , let go of the wheel, does it pull left, right, or does it go straight?
Also the info you should put into your signature is
Year and the trim line (LE, SE, SLE, SSE, SSEi)
these two items are extreamly helpful to anyone trying to diagnose your problems
Damien-they don't make them like this anymore
97 SSEi. 3.2mps,msd,optima,1.9 YTs,245 RE750s,SLP Headers,custom ported SC inlet/outlet, 180 deg, INTENSE pcm,42.5#injectors,255lph pump,custom soilid motor mount, 3.0 mps(maybe), and the nut behind the wheel.