Dear Kevin,
Regarding the parked condition, are you following my guidance for modeling idling rotors in FAST v7 in my post dated Mar 30, 2010 in the following forum topic: Extreme events? To model a parked rotor (where the generator is kept from rotating), you should further disable the generator DOF in FAST’s primary input file (GenDOF = FALSE).
Regarding the coupled springs model, based on the error, I would guess your model has gone numerically unstable. To simulate with the CS foundation, which has higher-frequency modes than the rigid foundation model, you’ll need to reduce the time step (DT) relative to the model with the rigid foundation to keep the model numerically stable.
Best regards,