Recheck the panhard rod bush-bolts then, along with all the other balljoints for security and wear. Check the UJs in the steering colum, check the colum for play. It will be harder to find if it's intermittant but chances are one day it will become regular, then you can get someone to waggle the steering to make it clunk whilst you go and feel all the steering joints unitll you find the ones that clunks.