Andy,
If your film is moving in the tank, it's an indication your agitation is too violent. Film should be stirred not shaken( as James Bond specified for his martini). When you agitate, the objective is simply to move away the small quantity of developer that's been in contact with the film for the previous 30 or 60 seconds, to replace with fresh stuff. Handle that tank as if it were a breast - not a cocktail shaker.