it's hard to diagnose this stuff over internet
you need to put the car up on a lift and see what's worn. suspension bushings and joints are major suspects but could also be struts, bearings or just a tire wear pattern in combination with some alignment setting.
generally, since it only happens at certain speeds it's most likely a resonance of some kind which involves things moving or flexing at a frequency which occurs at those speeds. you need to figure out what it is that's moving and/or flexing.
or you could just start replacing things in which case i personally would start with bushings, they are relatively cheap and are prime candidates for being the source of the problem. before you do that though, try swapping front wheels from left to right (don't leave it that way, just do it for a short run to see if the symptoms change). if things change then you might want to start with tires/alignment, if not then bushings or something else in the suspension is more likely.
I agree, put it on a lift and see what causes it.
lower control arm bushings go quick on...
these cars. Mine went out around 75k. not a do-it-yourself job, requires a press but replacing them with E30 M3 offset bushings improved my turn-in on an otherwise stock suspension.
my symptom was more like a delay on the turn in due to the back-end of the control moving about quite a bit. the ball joints on the lower control arm also like to go bad...
pay a pro, someone you trust to spend an hr under the car, they'll know best.
A similar thing happens to me
Only recently did this start happening to my 99 M Coupe. I have been getting this at speeds as low as 30 mph. My symptoms are temporary though as it goes away after a while. I haven't gotten it looked at yet. I have 130K mi on the car with a recent Dinan Stage 2 suspension. I thought my problem might be that the car is sitting for weeks at a time and the tires might be flat spotting. I took it to my first autoX and the 70 mile drive there seemed to cure it.