Occasional teeth grinding is not something I would worry about. but lets look at a couple of things. It is possible that there could be "caps" which are deciduos (temporary) molars that were not shed so that they dorm a cap which sticks to the permanent molar. This ceates a higher profile in the dental arcade and can lead to grinding. If so look for patterns of occuence, is it increasing over time, are there certain events during which the grinding is more frequent, does it have any relation to eatin?
Take a good look at each line of molars if you can and also let me know if there seems to be any pattern and I can discuss this further.
Thank you, Dr Rybka