Rear subframe.. Check that and have a mechanic look at it.. It is a common problem for this cars for the rear subframe to crack... My coupe has 111000km now and still no signs of the cracks.. Anyways, check out the link below it's mostly for the M Coupe but should apply some of them for the roadster.
2013 Ford Focus SE
2011 Lincoln MKT
2003 Escalade 8 Seater of Fun - 251k and still running
2003 Civic Si
2000 M Coupe