Yeh, Crichton is one of the few that I can remember which has re-used both Roma dressed and Roman carved blocks. I'm sure there's a Roman fort/signal station nearby which was obviously plundered! So, yes, Crichton is post-Roman. However, there are many others which I believe are genuine IA (older?) such as Pitcur in Perthshire which has, within it, two cup and ring marked stones;
http://www.themodernantiquarian.com/site/604/pitcur_souterrain.html
There are many otheres with cup/ring carvings, now, whether these were re-used stones of carved in-situ I guess we'll never know...