First thing i'd check tires pressure, and then alignment (if there had been any bigger shocks to car/suspension since times when you hadn't noticed such behaviour).
edit
lol, i should have refreshed before posting
P.S.
I assumed that after suspension mods you HAD done realignment? If not, highly suggested.
Another probability .. maybe you have changed tires to some with soft sidewalls? But given even lower/less flexible sidewall for tires on 18" then on 17" imho that's less probable.