Hi Bob,
The nodding donkey effect is not necessarily caused by stiff springs. I have had this and it was caused by high friction or stick-slip. If the bushes don't slide properly the car starts bobbing on the tires and there is little damping in them. Stick-slip is caused by wear or poor lubrication/greasing. Depending on the bushes installed and the usage, wear at 14000 miles is possible. I would say; check for KP wear and stick-slip and then decide if you want to change springs or KP&bushes or both. Cheers J
